I'm not familiar with the rotation of acuity thing. But if you work in a unit designated as an ICU, then it seems like most/all of the pt's would be critically ill and thus fall under "ICU care". Of course, different hospitals have different populations. Some hospitals have ICUs with 'less sick' --if you will-- pt's than maybe a hospital that does transplants or is a county hospital, etc.
As for just working with vents, well, I didn't know. There is a lot more to ICU than just vents. You need to get your experience on a floor with, yes, lots of vents
But also lots of drips like pressors/vasoactive drugs, lots of sedation, hemodynamic monitoring (CVP, Art lines, swans/lidco, etc.). You can have very stable pt's on vents, people go home on vents. You want the really really sick pts, and they will be in the ICU. As to which ICU is better, that really depends on the hospital. I work in a medical ICU and we happen to have lots of vents, multiorgan failures, drips, sedations, invasive monitoring, etc. Most people say SICU or STICU is really good, also CVICU. So when you're looking for a unit, try and see what kind of pt's they take because it's the experience with the pts, not the name of the ICU that will get you into school