Godzilla Vinyl Model Teeth Issue

Mitsdude is correct.  Vinyl is very difficult to work with, and not for beginners.  I make these two AMT vintage Star Trek figures and it was a real pain, as nothing fit right.  I had to use tons of putty to fill huge gaps in the body joints, essentially resculpting the figures.  But they came out OK.

Definitely heat the vinyl in some way. Trying to use a hobby (Xacto) knife on room temp vinyl models can result in part of you being removed.

Having said that you also have to be careful with heated vinyl as it can be cut like a hot knife through butter.

Also, vinyl is a lot easier to trim if you heat it up with a hairdryer or soak it in hot water first.

How about a pic of what you're talking about?  My guess is flash between the teeth.  You can trim vinyl with a hobby knife and a new, sharp blade.