I would assume that it'd be very important, but many people who I talk to tell me what jQuery or libraries that provide a similar API to it are everywhere, so there's not point in really learning how to do things natively.
- If they know a bit of jQuery and a whole lot of vanilla - This is your average JS developer. Hire this guy too.
- If they know jQuery but not vanilla - This person probably can get things done, but doesn't know how it works. Not optimal, but still a good guy.
- If they don't know jQuery but know vanilla - They probably use another framework. Ask first before it becomes...
- If they rejects jQuery but know vanilla - This person must be retarded, sticking to some stupid idea like "native is better than library.
- If they don't know jQuery and vanilla - This guy probably isn't a JS developer.
Your question is also backwards. You should get JS developers with jQuery/[other library here] knowledge as bonus. Just watch out for idiots who aren't open to other technologies though (like the fifth bullet up there). Just so you know, jQuery isn't the only library of its kind. There's Zepto, Mootools (and a few older ones). The guy you want is the one with enough JS and framework knowledge (one cannot really master everything) to move across frameworks.