we haven't taken any long time vacation( over a week),

but i heard what other people did is if the vacation goes beyond 2 weeks, you pay her half of normal wage. i.e. some families went back to china for 1 months, and the nanny was paid half. I think this makes sense since you need her when you come back and for her is involuntarily days off, which is unfair to lose the whole income. But this is just as a reference, as other people mentioned you should talk these in detail before hand, whatever arrangement should work as long as both sides are happy.