The daily survivor probability would probably provide the most useful information. My guess it would look something like a bell curve, sick enough to be put on a vent leads to a high chance of early death even on the vent, survive the initial weeks your odds go up of a positive outcome, I.e all the people with outcome unknown, stay on the vent for x amount of weeks head back down to a poorer outcome.
I think that's unlikely. In general, the longer one stays on a vent, the worse the prognosis. Being on a vent increases the risk of pneumonia, blood clots, sepsis, and a whole host of other very dangerous complications. After being on a vent 7-10 days, one usually requires a tracheostomy to help slowly wean off the vent and to prevent the risk of further infection. Typically, those that do best with a vent get intubated, turn the corner rather quickly, and then are extubated in a few days.