So what's the deal with the expression TONMAT VET? What does it mean here? thanks.

Tonmat vèt! (Tomat vèt!), that's Creole for Green tomatoes! 
As an expression, it's a pseudo swear word. People, who don't want to sound too obscene, might say it instead of Tonnè!, Tout Tonnè!, or even Tonnè de Dye! (Creole for holy crap, etc...)
Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words