meh -- no logical reason he couldn't have dealt with glads in the same way he did rachel by affecting his bio-energy. and while it's true he lost to glads (a nice showing for him for a change) he did better against the eldest than gladiator did and was ultimately the reason the eldest was defeated (the same eldest who pretty effortlessly busted gladiator up). don't forget -- vulcan's chrono age is THIRTEEN. he is written as impetulant because he's an angry kid and he is VERY inexperienced with his power.
given his powerset, he could have easily found a simple, logical way to beat glads. he didn't which was sorta disappointing. glads however was given some good respect in that mini as he was shown killing the shi'ar phoenix as well . . .
as for havok -- he owned havok several times (an impressive feat given some of havok's feats) until he was uber-amped. again, disappointing, but there is still a lot of time to see more of vulcan and where he ends up . . .
terrax would win now. vulcan has the potential to defeat him though.