valac is the Vala compiler. It's primary function is to transform Vala code into compilable C code, though it can also automate the entire build and link project in simple cases.
The simple case for use is:
$ valac -o appname --pkg gee-1.0 file_name_1.vala file_name_2.vala
-o switch requests that an object file is created, rather than just outputting C source files. The
--pkg option says that this build needs information from the gee-1.0 package. You do not need to specify details about what libraries to link in, the package has this information internally. Finally, a list of source files is given. If you need a more complicated build process, use the
-C switch to generate C files instead of a binary, and continue the process manually, or through a script.