the 1-2 days before vacation when you can't get much work done because your brain is already on vacation
I haven't been able to get shit done at work today. I have vacation brain.
When you feel like you're on vacation and you actually aren't.
Dude: Brooo forget the meeting, let's go to the beach!
Colleague: You've got a serious case of vacation brain. I have to get back to work.