The answer is: is. There are many assistents, but only one is on vacation. The correct form of verb that completes this sentence is the simple present of the verb to be, used in the third person of singular: he or she.
Simple Present of Verb to be
Infinitive: be
I am; she/he/it is; you/we/they are.
One assistent is on vacation.
The assistents are on vacation.