A TDR indicates a single rendering packet took 2 seconds or longer, either because you submitted too much work, or because there was a hardware problem resulting in a hang. VSync doesn't affect the duration of individual rendering packets.

And if you enable vsync, the GPU will tend to do less work because usually less frames will be rendered (this will be more pronounced in cases where unconstrained it would run at e.g. 500fps instead of 60fps). This can be important for battery life on portable devices, but as far as I'm aware shouldn't cause a GPU to hang (unless it has hardware issues like e.g. overheating).