Is there any way to subdivide an already created mask. I'm working on a project where I have already manually masked about 50 frames (about 2 hours' worth of work), when i realized that there aren't enough vertices (is that the word?) in the mask. Is there any way to fix this without just making a separate mask?
You should not be afraid of making more than one mask. It is a lot easier to rotoscope using multiple masks for different sections. Work with simpler shapes, they are a lot easier to manage than a single, larger, more complex one with lots of vertices (or control points).
Having multiple masks will allow you much finer control: you can turn some on or off, vary the feathering and have detail only where you really need it.
See how many masks are used in the following video: https://www.youtube.com/watch?v=NR9w2TRWRlI
You can combine different masks directly as layers
as done in this video https://www.youtube.com/watch?time_continue=482&v=5ZnJLLB1P4M
Or you can combine them in the compositor.
Use a color mix node to subtract, multiply or combine different masks in different ways.
Related: Separate Masks in Node Editor
Another tip is to use the tracking tools in blender and associate the trackers with point of the mask to automate the process.