I am sitting at my desk, right now, designing t-shirts. I combined 3 vectors on this list, and with appropriate text for the niche I'm targeting, I was able to make 50 shirts.
50 designs. Think about that. With only three vectors. And there are 37 other ones to use.
They can be combined with multiple vectors on one design, stacked together, large sized ones as a background, and so on. If you can think it, you can create it!