I think you have plenty of time (at least 5 months, from the sound of it, and possibly longer depending on when your next visit is) to work this out. In the grand scheme of things, unless the planned vacation is for an entire summer or something, I don't think it's that big a deal, honestly.
There's also the possibility that, while the distance is hard, being physically apart isn't as big a deal for your g/f as for you. I was in a long-distance (albeit 200 miles, not 2,000+) relationship, and my b/f was quite happy if we spent a weekend playing computer games together from our respective apartments, whereas I would have been way happier if we'd been in the same room. As a result, I was the one who made all the trips to him, suggested things to do in person, etc. For me, being together 3 weekends out of 4 was better, while he would've been OK with 1 or 2. Neither of us cared any less than the other (at first, anyway...there's a reason he's an ex
), but that was just our comfort zones.
Perhaps, along these lines, the concept of a 6-8 month wait between visits rather than a 5-7 month one isn't that big a deal to your g/f. Especially if it's a family vacation, which presumably she has less control over planning than if it were just her going. (This is along the lines of "well, if you're going to cut off my toe, may as well take the gangrenous foot as well. It'll hurt either way.)
You can/should bring this up as a concern if it's a big deal to you, but I think if you go in guns blazing and "why would you want to go on vacation before coming to see me?!" she will feel guilty, even if that's not your intent.