vc.lib holds the standard C library and things like integer math support. You'll have to link with this for pretty much anything you do. If you use the vc frontend and the default configuration files, then it will be done automatically for you.
amiga.lib is the Amiga utility library (not to be confused with utility.library). You can find its contents in "amiga.doc" in the NDK documentation. It has a huge amount of bulk in the form of library vector offsets, stubs, and addresses of hardware registers, which are only there for historical reasons, so only link with this if you use the utility functions.
msoft.lib, mieee.lib, m881.lib, and m040.lib are of course the math libraries to be used with -soft-float, -fpu=68881, and -fpu=68040/68060. You'll have to link with these pretty much any time you use floating point or math.h. If you link with mieee.lib then your program will also be dependent on the disk-based math libraries that come with Workbench.