You may take vacation whenever you like and as frequently as you like, as long as:

• You let me know ahead of time (preferably one month ahead)
• There are no imminent paper or project deadlines, or you are able to work during your vacation

If you are going away for a long time (a week or longer) and you would like to be payed during your vacation, talk to me ahead of the time. I will generally be willing to do this if you can propose some research work you will be doing while being away (e.g., reading some papers, writing papers or coding). Otherwise, if you want to just rest, do nothing, and not get payed, this is fine with me.

If the vacation is short (less than a week) simply go for it and work more when you come back or before you leave. Just let me know that you are gone and for how long.