The only way is to kill vineships, which you can't while you are allied, however you can make a privateer fleet (if you want a good fleet, just sell your ships to the market and then buy them back) and take out the vineships as they fly over to your systems, which will end up being tedious.

To get rid of the vines I am afraid you have to declare war on your unfallen ally.

Ah, Privateers are good idea. I'll remember that for next time.

I got annoyed and quit that game.