When a PhD program in the United States like this states that students are expected to engage in full-time research during the summer months and the January intersession in addition to their regular coursework, should one expect that vacation time is out of the discussion, unless it is unofficially granted by the advisor?
If your advisor is providing your pay, then you should negotiate vacation with your advisor. About 2 weeks per year is considered standard in US employment.
If your advisor is not providing you with pay, you should still discuss it with your advisor to make sure they think you are making progress and are committed to your research. However, your advisor should realize they are not in a position to dictate terms.
Do not be surprised if nobody cares. But don't abuse it.
I recommend planning your vacations at the same time as everyone else's. The benefits are that other people will not notice you are gone, and it can be hard to get work done anyway when the people you work with are gone so less is missed. Typically these times are late December and August.
My university is officially closed (skeleton crew only) from Christmas Eve through New Year's Day, and lot's of graduate students take this time off (and more). OTOH, there are plenty of profs that work their students to the bone and demand work over Christmas break, every other holiday when the university is closed, and every weekend, too.
If personal time is important to you, you should discuss expectations for time spent working or in the lab with a potential advisor before your sign on to work with them. My PhD supervisor expected people to put in appearances most weekdays in the lab and to be productive, but I know others that have Friday meetings that generate Monday deadlines.