Two meanings of "virtual DJ"
VirtualDJ, in one word, is the DJ program Atomix has published for years, which its makers call the most popular DJ software for working DJs. "Virtual DJ" in two words is also a plain description: DJing on a screen instead of on turntables. Searches with "online" in them usually mean the second, decks that open in a browser with nothing to download. That part exists, and the record above is the first piece.
What genuinely does not port
Real-time stem separation, timecode vinyl on real turntables, video mixing, karaoke and the range of DJ controllers a desktop program supports. VirtualDJ lists all of these; DJ Master Pro has none of the first four, and drives its platters from a MIDI controller you map yourself. If you are booked to play, the desktop program and its Pro licence are the tool, and the browser-based DAW page explains the technical limits of a tab.
A reasonable way to use both
Learn the hands in a browser, play out on the desktop. A mouse and a tab are enough for baby scratches, a first transform and a clean eight-bar blend on two decks; VirtualDJ's free use covers the next step at home, as long as no controller or mixer is plugged in. The cuts themselves have their own page on Scratch Master.