Older Children Sleeping in Parents' Room
We've had that and similar problems with our 5.5 year old. When he was 4.25, he was getting up 2 or 3 times a night. We were exhausted. What worked for us was giving him stars (or tickets or marbles, his choice) each night he stayed in bed all night. We had to explaing that coming in was okay in an emergency and then had to define emergency (his pillow falling off the bed is NOT an emergency!). After 15 stars, he turned them in for a toy of his choice. The toy was usually something I would not normally get him. We repeated this sequence 3 or 4 times and he's been in bed ever since. Now at 5.5, we are using this same technique quite successfully at getting him to go to bed without us sitting there until he's asleep. This whole process seems to work because we found a good motivator for him, he could tell our attitude had changed and we were serious about this. Regarding vacations, I guess that has to be explained as a special situation and all will be back to normal when you get home. We do that when I have to be out of town on business and my son sleeps with his dad those nights.
My son, now 10, has had periods in his life when he wants to sleep with us. After lots of attempts to get him to go to bed in his own bed and stay there, I decided that it was only making him more desperate to be in our room. So I bought a futon and told him he could sleep on our floor whenever he wanted to. He did for about month and then went back to his bed. In times of added stress (new school year, pet died) he will come get out the futon for a night and then go back to his bed the next night. It never effected his ability to go on sleep overs. For us, resisting his desire to be with us just made it worse.