Paul plainly explains for us what he means by "unless you believed in vain".
You can't be saved by a gospel about a risen Christ no matter how strong and sure your believing is if Christ really didn't rise from the dead.
"2by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain."
"14and if Christ has not been raised, then our preaching is vain, your faith also is vain."
"17and if Christ has not been raised, your faith is worthless; you are still in your sins. "
1 Corinthians 15:2,14,17
No amount of believing in Paul's gospel of a risen Christ (vs.3-8) is going to save anybody if Christ has not really been raised from the dead. But osas ignores the context provided right in the chapter and says "unless you believed in vain" means you are saved by the gospel unless you didn't really believe, and because you don't really believe you won't hold fast to the gospel like true believers do. But as we can see in this post and my last post that's not the message Paul is communicating.