All of the responsibility designation and time allotment should be discussed and distributed during the planning phase. With proper communication no ones vacation plans should be a surprise. However, there are personal problems that come up. If the teams vacations are discussed during planning then there work will be completed before they leave or after they come back, depending on when their task deadline is. If they will not be able to complete their task when it is needed then they should bow out and have someone else take their place.
When they are in the execution phase and something unforeseen comes up then depending on how long their absence will be they can work from home, be replaced, or split their work among other team members until they return. With proper planning their should not be an issue during execution when it comes to team member absence.