An important point to understand, though, is that taking a vacation for the entire summer is not really an option, no matter how few grants you have. You're expected to do more than that over the summer even though supposedly you aren't paid for it.
Others who are more senior and less research active may shoo their students off to internships (particularly in CS) and spend the summer doing very little. A lot of departmental committee work is paused, and people tend to know who is and is not available over the summer and will make arrangements accordingly. (I.E., don't assign Professor Tee Time to a committee that needs to function over the summer.)