A couple of choices:
Assuming you are using Project 2010, in the Gantt chart view, right click on the overallocation indicator, and select "ignore problems for this task".

A better answer (in my opinion) is to make sure you have tracked the past tasks. Assuming the resource did not work during her vacation, enter zero for the actual hours during her vacation (Task Usage view) and the overallocation message should disappear for that task. (Assuming she is not still overallocated in the future).