Just some more VP string data points from my trials and tribulations...
DR Silver Stars (essentially the same as Black Beauties but Silver) - E string seemed floppy, sucked too much top end out of the bass. Otherwise good and smooth with nice bottom end. Can get lost in the mix.
Helbourg Strings - plenty aggressive sounding, not as smooth on slides as I'd like, a bit too much top end for my tastes. Probably a good string for a pic player in an 80's band. No problem cutting through a mix.
Cobalt Flats - nearly perfect in every regard with this bass. Probably the best flat wound for the VP if you're not looking for a pure traditional Roto type flat wound sound. More grit and growl when wanted, more top end when wanted, more complexity in tone than Rotos, yet still very deep and smooth on the bottom, well balanced mids. Some say they have a bit more grip than most flats, feel about the same to me. You will be heard loud and clear in a mix.
Fender 7150 Nickel Rounds - so far my favorite round wound on the VP. Delivers nearly everything the Cobalt Flats do but with a true round wound sound (the Cobalts are almost halfway between a round and flat in sound). Perfect top end, deep rich bottom end, nice low mid punch. Very versatile on the VP, can cover full round wound range and as the tone knob is rolled off even does a decent job of inching into flat territory. Sounds great in a mix.
Roto Swing Flats - very traditional flat wound sound, kind of low and mid range focused. Not a great top end but ok. Feel great, well balanced nice tension. Just a middle of the road string on a stellar bass.
That's it for now, as always, this is a very subjective topic. With different ears, hands, amp, cab, playing style and environment you might get completely different results. But the information is probably directionally correct at least.