A VR game is fine as far as I know. Like you noted it will be difficult to find many people willing and able to play your submission and I feel that one of the great benefits to participating in a game jam is to get feedback on what you've made. I remember last year there was a multiplayer only game which the developer for had trouble getting people to play, but it's certainly not against the rules to make a game like that. The only problem I can think of is that one of the jam organizers will need to be able to test it, to confirm it's a valid submission (for example, games which don't run at all or are found to have malware are removed.)