Can I insulate over 1/2 in. styrofoam?
1/2" Styrofoam will give you about R-2.0 to R-2.5 insulation. The best you can expect from any reasonable 1/2" material is R-3.0 to R-3.5, so it's not too bad for what it is. Obviously you will get much better R-values from much thicker material.

Vapor barriers should only be installed closest to the warm side of any insulating assembly and nowhere else. While Roxul is largely water resistant, you still do not want water condensing inside the insulation, it reduces the insulating value and promotes mold growth. You also do not want to block any moisture that gets this far from being able to escape by having additional vapor barriers farther in from the warm side.

So you must remove the existing vapor barrier. Never install insulation on the warm side of a vapor barrier. Unfortunately, styrofoam also serves as a vapor barrier, so you should remove it as well before installing the Roxul. Better to give up that R-2 than risk moisture being trapped in the Roxul. The one and only vapor barrier should be on the warm side of the Roxul.

Note that this is assuming there is no risk of water ingress through the basement walls. If there is any risk, you should be using an insulation impervious to water such as XPS, and some positive drainage scheme must be in place behind the insulation to allow water to drain to a sump for removal.