Will the Beachtek DXA-4p work with the sony vx2100? If not, why?

Yes, it will work. The only difference is the location of the mounting screw, for ideal balance and access to the adjustment knobs. The VX will sit a bit further back on the 4p then on the 4.

Thanx David, I received it yesterday and tried it out. Now just need to purchase my cables.