Is it possible to accrue vacation hours during graduate study as a research assistant (GRA)?
The answer is: it depends.

The biggest factor is where you're a graduate student. For instance, graduate students in Germany receive up to six weeks of vacation per year, while graduate students in the US may only get two weeks' vacation per year, if even that much.

However, I think that in the US, where policies are a bit less generous, the real deciding factor is your graduate advisor. I think that reasonable graduate advisors will permit students to take off a few weeks per year (especially since most of them might do so themselves!), provided that you make sure it doesn't conflict with any major deadlines or events in your group. For instance, asking to take substantial vacation time just before your first big conference is probably inadvisable. But time during the holidays or during a relatively "quiet" time in your group (whenever that happens to be) would probably be OK.