Legal Question in Real Estate Law in Washington

That's up to your neighbor. If the neighbor has no issue with it, then yes, go for it. If the neighbor cuts your vines at the property line, regardless of where the fence is, that is their right as well.

Interestingly enough, if you keep your vines there for a period of 5 years, you may be able to obtain what is known as a prescriptive easement, or better yet, you may be able to claim that land belonging to your neighbor up to the fence, by adverse possession.

It's a long shot, and not one you should count on, or try to do, since it will forever kill any good relations between you and your neighbor (assuming that is important to you). Your best option is to talk to the neighbor and make sure they are in agreement with what you want to do.