For vacations specifically, it helps a lot for a PM to talk with each team member they’re managing and perhaps get a heads up on when exactly they may take a vacation. This will help you avoid conflicts in the foreseeable future. For example, if someone lets you know that they’re deciding to take a vacation during one of the most crucial parts of the project timeline, you could have a chat with them and possibly get them to reschedule their vacation, or maybe focus their efforts on the crucial task. This is where your relationship with your team members will come in handy and be a deciding factor in how successful you are with this persuasion.