on vacation
Spending time away from work or school, especially to travel some place for recreation. Primarily heard in US. I am planning to be on vacation for two weeks in July, so if you need any help during that time you can contact my colleague Sarah. We're going to Disney World on vacation this summer!
on vacation
away, taking time off work; on holiday. Where are you going on vacation this year? I'll be away on vacation for three weeks.