Do U.S. PhD students who engage in research during summer or intersession breaks get vacation time? 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.