Well, if you successfully killed the Destroyer, and did the final quest after it, then when you play the game next time there should be an option for Playthrough 1 or 2. If you go to the 2nd one, the game starts over, but you keep all your guns and items from this 1st playthrough. If you choose the 1st playthrough, then you'll be back to where before you killed the Destroyer. And I apologize in advance if this doesn't work. I myself haven't beat the game, but my friend told me what happens.
If the problem is entering the Vault - travel to the Vault through Fast Travel/New-U stations. If it's about the Destroyer not showing up for fight than it's a known bug in the game (there are workarounds): http://borderlands.wikia.com/wiki/The_Destroyer