Well, the hexen/heretic source is written in C/C++ so there's no choice to that.
Now that I think on it, if you can download the Vavoom progs source you won't have to mess with much stuff, like libraries, the thing with Vavoom is that you can modify the progs without having to use a C compiler, it has it's own compiler and the language is powerfull enough to let you change most of the stuff in the game easily, also, there's some documentation on the basic functions and language structure in the
Vavoom's Homepage.
Now, if you ask me where to start I would say to download and decompress the progs source and then read the documentation there and once you understand most of it, you can start with some easy change to the game <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->.