15 MB is nothing. It has to be beyond ancient to have that little (meaning 16 MB). Almost everyone, and most "old" computers can easily spare 64-128 MB guaranteed. Since it sounds like you'll be far below 64 MB, you don't need to worry about it. If you want to go with 128, you should have no problems either.
Also, "new" GPUs...well, have at least 1 GB...
Finally, do some research for statistics on this. The Steam Hardware Survey might have VRAM stats, but those will be skewed much more towards computers with dedicated GPUs (which most people don't have). Most people have integrated GPUs which share system memory.
Edit: Remember, GPUs with only 16 MB of VRAM (shared or dedicated), might not even be able to run your game anyway. You should be more concerned with GPU optimization as it's so easy to make it too hard to run.