First, that's not a "when", but an "if", and a big one.
We are far from the only good option (winks at preact et. al.), so let's see how this all plays out ;)
that being said, of course this would be a big boost for the Vue community. But one thing to keep in mind is that this would foremost concern the backend - people can and will use whatever they want on the frontend, so we wont achieve world dominance (yet).
From what we can see, the WP team is very careful to try and find a solution that is highly adoptable for everyone interested in contributing or extending wordpress, so personally I suspect that they will take their time in finding something that fits that bill, and it might no necessarily be a plain-and-simple 1:1 adoption of any of the frameworks out there.