Communication is really the key for all project teams. Both communication between team members and communication between the team members and the project manager are essential to the performance of the team. In terms of taking vacation as you mentioned, this should be settled at the start of the project, knowing when team members are going to have to take off, and need to be replaced. However, life isn’t predictable, like you stated, these are humans after all, not robots. So if something comes up, it should also be prematurely planned for by having people who can either pick up the slack or replace the person. In most circumstances, communication between the people working on a project can overcome any situation like vacation days and still have plenty of success.