What are vac words in spelling?

There's no such thing as "vac words" in spelling. It sounds like you might be referring to a specific spelling concept or term that's been slightly misremembered.

To help me understand what you mean, could you please tell me:

* Where did you hear the term "vac words"? Was it from a teacher, a textbook, or another source?

* What do you think "vac words" might mean? Are they related to a specific type of spelling rule or pattern?

Knowing this information will help me figure out what you're asking about and provide you with a helpful answer.