Eva!

“Captain, it’s just a model…”

I did think of one specific thing that the Borg are significantly better than the Empire at: time travel. Everyone and their mother in Trek does it, and the Borg do it while on the run in First Contact. Meanwhile, (spoilers for a show that ended 6 years ago) the Emperor’s desire+inability to control a force-based time nexus is a subplot in Rebels to the point where he decides his best option is to try parleying with the heroes. So, if Vader became a persistent threat the Collective’s best chance would be to zip into the past and kill/assimilate child Anakin./

All that said, any ideas as to how this might work really falls apart when you look at temporal mechanics across universes. Trek canonically has a fluid (though resilient) timeline, this was stated onscreen in SNW. What little we see in Rebels indicates that Star Wars has a single stable timeline (one character survives a duel in Season 2 this way), and you can argue that this squares with Force precognition (including most notably the clear and unambiguous visions courtesy of the Mortis Gods in TCW).

Definitely fair, the Borg are on the low end of scary as far as space zombies go and I’d say at that point we’re kind of running up against the fact that the Force does what the plot needs.

Vader at the peak of his power is pretty crazy. IMO it’d take a lot of drones to overwhelm him in a direct confrontation, but a cube has drones in the tens of thousands, so that’s at least in the realm of plausbility.

Most interesting cross-universe interaction is if the Force can be used to resist transporters, because spacing Vader is probably the best way to get rid of the threat. I think that’s a moot point though since the borg can (and do) blow up their own ships to eliminate even minor threats (see: the Borg Queen blowing up a cube of 64k drones for a couple deviants in Unimatrix Zero). So, their best chance is to transwarp to the middle of nowhere and self destruct the cube he’s on. If the ship’s detonation doesn’t take him out, just count on the cold equations of space to do the rest.

Conclusion: Darth Vader would pose a grave threat to any Borg facility he should choose to board, but the Collective is resilient enough to not really care about any damage he could do.