"Trap sound"? Not quite sure what you mean. If you're looking for trap styled sounds, I would suggest searching some tutorials on youtube with Massive or Sylenth, but there's no actual "Trap VST".
As far as I know, there is no trap VST as a standalone tool. You'd have to use a combination of vst's and instruments. What I find useful are orchestra soundfonts (many can be downloaded free from Warbeats.com) and also Nexus has many good quality synths that mimic orchestra instruments. Furthermore, try purchasing stuff from PrimeLoops - I know they have several trap/dirty south kits that can help you get the sounds you want.
I make a lot trap beatz, I love to use Philharmonik for Orchestral, I just got my hands on EastWest Symphony Gold Edition which is soooo sick, I love the brass! I use Nexus a lot, having some hot expansions to buy and Sylenth is unbeatable as well!
You will find sometimes that real sounding strings are not what you want. You actually want like a synth type string patch.......the corniest soundfonts will do. The cute pretty well known vsts will give you a ......cute...pretty sound.
Zaytoven dead gummit. The hardest of all of them. Lex been soft....real talk.