Decent vector library for C?

The standard Microsoft SSE wrapper library is DirectXMath. Is that the sort of thing you're looking for?

If not what functionality are you after?

I prefer eigen. It's header only, fast (heavily vectorized and lots of template magic to make all operations efficient), packed with features and works everywhere. There are some quirks to be learned, because it's more like a math library, rather than a gamedev / graphics one.

Depending on what you want out of it, there are a number of possibilities here as well:

https://github.com/nothings/stb/blob/master/docs/other_libs.md

+1 for DirectXMath

Thanks guys

I'll check them out

Both DirectXMath and eigen are C++ libraries. Since you specifically asked for something in C, you might try Kazmath.