I've been using omni complete with ctags for the intellisense-ish feature in Vim. I also have been using the STL tags db, but it only works with files that have the .cpp extension. I suppose I can just change everything from .c to .cpp? Is there a command perhaps that I can use while creating the tags to avoid having to do this?
What exactly is it that only works with .cpp files? That's probably OmniCppComplete only being turned on when it thinks it's relevant. There's probably a command you can add to vim's filetype handler; if you poke around in the .vim directory, and find where OmniCppComplete is turned on for C++ filetype, you can do the same for normal C. I don't remember where it is exactly; .vim/after/ftplugin.vim? Something like that. But first, is that indeed the problem? I'm thinking this has nothing to do with the tag file itself, but with the plugin.
What Quix is describing is something that you can put in files to tell vim how to handle it on a per-file basis. You can do things like set the filetype, indentation, etc. The approach where you edit ftplugin makes it global. If you truly want all .c files to work with OmniCppComplete, you probably want to make it turn on the plugin when a .c file is opened.