Generating a clock signal in the VWF

If you're learning VHDL, I would recommend taking a look at learning VHDl testbenches. With a vector waveform file, you have very limited testing capabilities, in that you run data through it and see what comes out the other side. For some applications this may be all right(like video editing, where you just dump a video feed into it), but for things more complex, a testbench lets you write higher-level functions that actually make decisions based on what the DUT(device under test, i.e. your source code) does.