Would a pilot flying in a flight simulator with VATSIM make a mayday call in a scenario in which a pilot in a real plane would?
In a real Level D simulator, you perform for the instructor as it it was real life and that includes radio calls. Unless the instructor tells you not to bother.
For VATSIM, you should go on the controller's VATSIM discussion board and ask if emergencies are encouraged. Some of the controllers on VATSIM are real life controllers and would probably enjoy the change of pace, and so would most of the other flight sim participants for that matter. But I would check with the community first.