I suppose all any of us can tell you is how it worked at our schools, so.... best to talk to upperclass peeps at VMCVM. But the first thing I'd say is that you shouldn't worry about it too much and that you'll have options.
For us, we put in specific dates for our externship and vacation rotation requests and those were supposed to be honored. And, for the most part, they were. So you likely will be able to schedule vacation over it. On top of that, some rotations are weekend-free and some aren't, so if the vacation plan doesn't work out, you could try to schedule a rotation that at least frees up that weekend. If that doesn't work and you somehow get screwed and end up on a rotation with weekend duty, most of the time it's possible to work out something with a classmate to have them cover your cases in exchange for you helping them out on another weekend or whatever. Last ditch, you could always talk to the rotation chief of whatever rotation you end up on, explain the situation, and ask if there's any way to make it work.
All in all, I wouldn't spend much energy worrying about it. 4th year scheduling is intense and sometimes unforgiving, but for important things people manage to make it work. You'll get to the graduation and the party. You may not have a ton of time to plan the party and be the traditional host, but it will work out.
