The problem is that you're seeing something other than someone else. Such as the fact that you look like you're driving, say, a Cheetah to everyone in the game, but the physics and dimensions are different on your game. Therefore they may see you stuck somewhere in the hood of the car, causing their game to crash. The easiest solution is to create two installs of Vice City, one unmodded for MTA, and the other for all of your modifications.