I'm sure you've already checked to see that nothing is between the back of the seat and the rear floor. If real hard slamming won't do it and your Vibe is still under warranty, take it to your dealer.
dont worry folks after halve hour of frezzing my (removed) off wiggling and jiggling with a little bit of cursing I tried a liberal amount of wd 40 a bingo presto she locks up just fine . must of been a bit a rust along the bar the latch is attached to.Ian