Let me begin by giving us three of the definitions for vain.
Vain: Empty; worthless; having no substance, value or importance.
Fruitless; ineffectual.
False; deceitful; not genuine; spurious.

1 Corinthians 15:[1] Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
[2] By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
(False; deceitful; not genuine, spurious)
There salvation is not based on not forgetting but on whether or not they truly believed or were pretending to have believed. The only person who knows whether or not they are a Christian is the person themselves. Being a Christian is a personal decision and based on the internal workings of the heart. It is based on the acceptance and belief of the gospel of the grace of God.
If there is no Resurrection then our belief is vain.

[3] For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
[4] And that he was buried, and that he rose again the third day according to the scriptures:
[5] And that he was seen of Cephas, then of the twelve:
[6] After that, he was seen of above five hundred brethren at once; of whom the greater part remain unto this present, but some are fallen asleep.
[7] After that, he was seen of James; then of all the apostles.
[8] And last of all he was seen of me also, as of one born out of due time.

The gospel remains unchanged! The gospel of the grace of God given to the Apostle Paul and through him given to the church has remained unchanged for almost 2,000 years and is still being preached to the saving of men’s souls.

[9] For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God.
[10] But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain;
but I laboured more abundantly than they all: yet not I, but the grace of God which was with me.
(Fruitless; ineffectual)
[11] Therefore whether it were I or they, so we preach, and so ye believed.

Jesus Himself said the greatest Apostle would be the least and indeed the Apostle Paul was the last one to have a personal and physical encounter with the Resurrected Savior. Indeed he was the greatest. The grace of God was not fruitless or ineffectual in the Apostle Paul but the very basis of our knowledge of doctrine and the will of God in the everyday life of a Christian come through the Apostle Paul. The person who preached the gospel to you did not save you, it is the belief of the gospel that saves the soul.
If there is no Resurrection then our work is vain.

[12] Now if Christ be preached that he rose from the dead, how say some among you that there is no resurrection of the dead?
[13] But if there be no resurrection of the dead, then is Christ not risen:
[14] And if Christ be not risen, then is our preaching vain, and your faith is also vain.

(empty; worthless; having no substance, value, or importance)
If there is no resurrection then what in the world are we believing in? Why would it matter? If when we die that is all there is it would not matter what we did or did not do in this life. If your Savior is dead then you have no Savior. What you have is a false religion. I can take you to where the body of Mohammed deteriorated and lays but not so with Jesus. His body was gone! He is alive today and currently sits at the right hand of the Father. His resurrection proves that God will raise the dead and we will have a life after this one.
If there is no Resurrection then our religion is vain.

[15] Yea, and we are found false witnesses of God; because we have testified of God that he raised up Christ: whom he raised not up, if so be that the dead rise not.
[16] For if the dead rise not, then is not Christ raised:
[17] And if Christ be not raised, your faith is vain; ye are yet in your sins.

(empty; worthless; having no substance, value, or importance)
Christ is at the right hand of the Father making intercession for us. If He did not rise then we have no intercessor. If we have no intercessor then we still have our sins.
If there is no Resurrection then our religion is vain.

[18] Then they also which are fallen asleep in Christ are perished.
[19] If in this life only we have hope in Christ, we are of all men most miserable.

If we have our sins then we have no hope.
If there is no Resurrection then our hope is vain.

[20] But now is Christ risen from the dead, and become the firstfruits of them that slept.
[21] For since by man came death, by man came also the resurrection of the dead.
[22] For as in Adam all die, even so in Christ shall all be made alive.
[23] But every man in his own order: Christ the firstfruits; afterward they that are Christ’s at his coming.
[24] Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power.
[25] For he must reign, till he hath put all enemies under his feet.
[26] The last enemy that shall be destroyed is death.
[27] For he hath put all things under his feet. But when he saith, all things are put under him, it is manifest that he is excepted, which did put all things under him.
[28] And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.

Not only has Christ risen but because He is risen so shall we also be risen. Because He is risen He shall also rule. Because He is risen He is able to destroy Death. Because He is risen He can overcome all enemies.

[29] Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?

If Jesus is dead then we are baptized for the dead.
If there is no Resurrection then our ordinances are vain.

[30] And why stand we in jeopardy every hour?
[31] I protest by your rejoicing which I have in Christ Jesus our Lord, I die daily.
[32] If after the manner of men I have fought with beasts at Ephesus, what advantageth it me, if the dead rise not? let us eat and drink; for to morrow we die.

If there is no Resurrection then our very life is in vain.

[33] Be not deceived: evil communications corrupt good manners.
[34] Awake to righteousness, and sin not; for some have not the knowledge of God: I speak this to your shame.
[35] But some man will say, How are the dead raised up? and with what body do they come?
[36] Thou fool, that which thou sowest is not quickened, except it die:
[37] And that which thou sowest, thou sowest not that body that shall be, but bare grain, it may chance of wheat, or of some other grain:
[38] But God giveth it a body as it hath pleased him, and to every seed his own body.
[39] All flesh is not the same flesh: but there is one kind of flesh of men, another flesh of beasts, another of fishes, and another of birds.
[40] There are also celestial bodies, and bodies terrestrial: but the glory of the celestial is one, and the glory of the terrestrial is another.
[41] There is one glory of the sun, and another glory of the moon, and another glory of the stars: for one star differeth from another star in glory.
[42] So also is the resurrection of the dead. It is sown in corruption; it is raised in incorruption:
[43] It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power:
[44] It is sown a natural body; it is raised a spiritual body. There is a natural body, and there is a spiritual body.
[45] And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit.
[46] Howbeit that was not first which is spiritual, but that which is natural; and afterward that which is spiritual.
[47] The first man is of the earth, earthy: the second man is the Lord from heaven.
[48] As is the earthy, such are they also that are earthy: and as is the heavenly, such are they also that are heavenly.
[49] And as we have borne the image of the earthy, we shall also bear the image of the heavenly.
[50] Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.
[51] Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,
[52] In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
[53] For this corruptible must put on incorruption, and this mortal must put on immortality.
[54] So when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory.
[55] O death, where is thy sting? O grave, where is thy victory?
[56] The sting of death is sin; and the strength of sin is the law.
[57] But thanks be to God, which giveth us the victory through our Lord Jesus Christ.
[58] Therefore, my beloved brethren, be ye stedfast, unmoveable, always abounding in the work of the Lord, forasmuch as ye know that your labour is not in vain in the Lord.

But Praise God there is a Resurrection!!!! Our belief is not vain! Our work is not vain! Our religion is not vain! Our hope is not vain! Our ordinances are not vain! Our life is not vain!