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.
“Captain, it’s just a model…”