Let me just say that your Fuller is a light duty vise. You could theoretically drill a hole in the mounting plate and add another hold down bolt, but if I were you, I would look for an old vise of higher quality if you need to do serious work. Drilling on the Fuller could easily crack the thing making it unusable.
Looks like you are cutting maybe 1 1/2 inch or bigger steel pipe. You would be best served by a vise like this one. If you keep your eyes open you can find them cheap, I bought this one for $15