It's not vanity that you can KNOW you're saved. "But God, who is rich in mercy, for His great love wherewith He loved us, even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved:) and hath raised us up together and MADE US SIT TOGETHER IN HEAVENLY PLACES in Christ Jesus". (Ephesians 2:4-6).
Before He promised us this heavenly position, He placed a seal on us assuring us of life forever with Him.
"That we should be to the praise of His glory who first trusted in Christ. In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also AFTER THAT YE BELIEVED YE WERE SEALED with that Holy Spirit of promise". (Ephesians 1:12,13).
"Having forgiven you ALL trespasses".(Colossians 2:13).
If you are saved, you are already seated in heavenly places IN CHRIST. Not only that, the moment you believed, you were sealed with that Holy Spirit of promise. This is like the candle wax melting onto the King's message and his signet ring making its mark. No one could unseal the message except the person to whom it was sent. If another tried, the seal would be broken, and there would be no guarantee that the king's words were his and not another's. This is so important to understand. You are sealed unto the Day of Redemption. The seal cannot be broken until it is delivered unto the recipient, Jesus Christ.
It is NOT vanity to be assured of salvation. You cannot be an effective workman for God if you are constantly worrying about your own salvation.
I don’t worry about my salvation. I trust in Christ’s Mercy.
I just don’t believe that Scripture can be ignored in regards to our actions as Christians. We have obligations, even as sinners, to live as He taught us.