I imagine the only limit would be finding a VR game that has enough stress, and playing it for long enough. Most people probably wouldn't willingly play a VR game that stressful for that long.

You can get PTSD working as a rather on offensive content, that’s for sure.

Does PTSD require elevated stress levels during the trigger event?

Feeling trapped is part of the trauma of PTSD if I recall correctly. So it should probably be harder to get PTSD from VR war games from being capable to retreat at any moment. I would advise caution about promoting "engagement" in a VR war game specifically to avoid creating feelings of obligations which could leave the player "trapped". Sounds stupid admittedly, but the damage PTSD does can be real.