I think it’s pretty much infeasible. Some professor did it in a lab once but I can’t see that level of sophistication being used against a public vpn server, whatever the circumstances.
SGX encrypted memory would work as a solution for this, probably.
It's been awhile since I read about this, but I remember the discharge time of the tested ram was so slow it seemed like anyone reasonably quick and prepared could potentially pull it off, even without freezing. But it was with SDR or DDR and modern ram discharge is so much quicker it is basically impossible now.
SGX encrypted memory would work as a solution for this, probably.