And now the MST3K clip:

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The earliest I can think of was the Master Brain computer in the second Doctor adventure The Mind Robber, which occupied a white void outside of time and space, and was capable of giving physical form to objects and characters thought up by it's human operator.

You could argue against it being VR because the action took place in a pocket universe, rather than a digitized computer environment, but given recent theorizing about quantum computing, perhaps this episode was even further ahead of the curve than Deadly Assassin.

Likewise, you've also got the Miniscope in Carnival of Monsters, and later, the CET machine from Nightmare of Eden, both of which miniaturize small chunks of physical space, and store them on computer chips.

Again, this slightly bends the definition of VR, since the environments are real, but held in an electronic "buffer" (which probably makes them closer to the Enterprise's transporters than anything else)

Finally, during the unfilmed 23rd season, the 6th Doctor was to have reencountered the Celestial Toymaker, operating out of a Blackpool fun fair, who had developed a new fully-immersive video game that killed players for real once they used up their allotted "lives." (No word on whether the crystaline Space Invaders-themed monsters would then spend the next several seconds teabagging their victims' corpses and compulsively rifling through their pockets for loot)

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