Is it possible that some of the kids will come, but have just not RSVP'd? Did you ask for RSVP's? Parents are notoriously bad at doing it, so I'd not rely on that to tell you who will be there. On the other hand, as a mom, I would definitely want to be prepared for the possibility that none of the kids will come. Unfortunately, this is one of those times that your daughter will have to experience her disappointment. And probably all you can do is comfort her through it, try to distract her from it as much as possible, and DO NOT make the mistake again of inviting schoolmates to a summer party for this very reason. Since you aren't seeing the kids every day, or the parents, it is very easy for parents to brush off a summer party. And legitimately, like you said, many families are on vacation, and if they're anything like me, just very busy with a ton of other summer activities. No excuse for not going to a party, but it may be for some of them.
If you do decide to have a party for friends as opposed to family only, I'd limit it to any close friends, so you can be slightly more sure they'll come, but even then, it's no guarantee.
Lastly, I know this is not the BEST way to deal with your situation, but for me, if it were my son dealing with the disappointment, I'd probably offer him something in a lame attempt at consolation. Perhaps you might suggest to her that if her schoolmates aren't available for her party, maybe the 2 of you could do something a little bit special to give her a little pick me up. Maybe take her to a salon to have her nails painted, or to the coffee shop for a big girl treat, whatever she would find special. I wouldn't be able to help myself but to try to lift my son's spirits with something a little special.
Best wishes to you and your daughter. This is really hard. I hope that some kids DO come, and your daughter will be happily surprised.