Is it ok to take a "vacation" from femara (letrozole)?

Discuss with you MD. It depends on what you mean by "vacation". How long you are thinking to stay off the femara (letrozole)?. Since i don't know any other information- such as the staging- the goal of therapy, your tolerance etc, i would suggest you to discuss this matter with your oncologist.
Depends why. The answer will depend why you are taking it. If you are using this for infertility and ovulation induction then yes it is ok. If you are using this for breast cancer then i would have to defer to the oncologists and breast surgeons for a reliable answer.