Legal Question in Real Estate Law in California

First, let me say I sure hope you can get this worked out without rancor or a lawsuit. Bad enough to be in an on-going dispute with someone; worse if they live next door.

As to the legal implications: If the fence is not on the boundary, but is instead a short distance into the neighbors' property, e.g., six inches, and they built it at their sole or almost-sole expense, I'd think a court likely would hold that it is their fence, and that they can prevent you from attaching vine supports or allowing your vines to grow on it, which would be a technical trespass, although they'd never get more than nominal damages. It is also possible, although somewhat less likely, that a court would rule that, notwithstanding its location and the payment of all (or almost all) of the costs by the neighbor, it is indeed a boundary fence and you are entitled to use your side of it as your own.

I do not know of any cases directly on point. You might want to think about some kind of trellis or stakes-and-wire arrangement for your vines, keeping it on your side of the property line.

If there is a drainage problem this winter or in the future, that raises a different set of issues, and at that point you might want to contact a drainage engineer and/or lawyer to examine the situation and advise you whether it was cause by the neighbor's grading work.