The four pads on each deck have different modes (Hot Cue, Loop, Sampler, and Effect). In Virtual DJ, you can customize what these pads do. Try setting one mode to Stems so you can toggle vocals on and off during a transition.
The Party Mix has only 4 pads, but with a "Shift" button, you get 8 functions. Here is a recommended custom map for Virtual DJ: virtual dj numark party mix