Post-trip update: Long story, but I ended up diving with both Subtropic and Southpoint Divers during my week in Key West, so I got 3 dives in on the Vandenburg and 1 on the local Reef.
Subtropic:
My first experience with the guy manning the front desk when I went in to sign up was not great (tough to get straight answers on whether they recommended a guide for someone who's experienced, but not with the wreck, and on equipment and conditions questions) However, the experience on the day of the dive was great. Very knowledgeable captain, crew and dive con. All 5 divers were advanced cert, so they recogized this and allowed more flexibility in dive plans. I buddied up with a similar experienced diver as there was only one guide on the boat and he took the husband/wife couple who were a little more nervous. 2 great dives on the Vandenburg this day with no current and 40-45+ vis. We were moored at the 3rd buoy and for the first dive explored all the way to the bow, 2nd dive explored all the way to the stern. Lots of large barracuda, Jack, reef rish, bait balls and a Huge Grouper playing hide and seek in lower decks. Took a little while for them to get the boat started for the return trip, but otherwise no problems. Definitely would use them again.
Southpoint:
Much more polished front-desk operations and a larger boat. These folks ran a very safe, if not overly conservative operation. I was discouraged to dive Nitrox by the guide. The dive guide assigned to me and another diver took us in a circle around the middle part of the ship and limited the dive to only 17 minutes bottom time. Vis had dropped this day to about 25', but this was disappointing not to get further on the wreck. 2nd dive for the day was a nearby reef. Visibility was 20' and there was a lot of surge and turbidity which made it difficult to navigate. No guide on this dive, we ended up surfacing after a leisure 45 min dive well away from the boat, but an easy swim back. After each dive the captain reviewed everyone's computer to make sure no one went into DECO and had 500+ psi.
So, my recommendation is if you're a newly certified or nervous O/W diver, Southpoint could be the way to go. If you're an experienced diver looking to dive Nitrox and maximize bottom time, Subtropic is my vote.
Great dive site. I'll post a video link shortly.
E.