No one can give you an answer to this other than yourself. You need to ask yourself - What do you wanna be? What do you wanna do? What do you like?
Typically these are the areas you might want to consider specializing in when it comes to 3D:
- Rendering (which is where vray and mental ray come to play)
alternatively, of course you can go ahead and learn all of these things!
If you want to become an architect you may want to consider Modelling, texturing and lighting, and possibly rendering. (Although for architecture there are better solutions than 3DSmax, such as AutoCAD)
If you like Disney/Pixar films like Toy Story you might want to consider Modelling and Animation.
The same thing also applies to game development (except maybe lighting and rendering). Game characters need to be modeled, they need to be textured, they need to be animated. Whichever one you want to learn to do is up to you
Also, 3DSMax does not specialize in any one of these areas. It's great for all of them.