From this thread https://texags.com/forums/84/topics/3209857/last, Marcus Aurelius said 90 +/- 5% mortality for patients on ventilators. I'm not sure if it is just his own personal experience at the hospitals he is at.
my hospital system had 55% mortality for patients on vent during the last surge. Mortality roughly correlated with age (ie 60% for those in 60's, 70% for those in 70's, etc)