Despite the Vantec NexStar dock using an Oxford 934DSA bridge and the NewerTech Voyager dock using an Oxford 934DSB bridge (note A vs. B), applying the firmware update provided by NewerTech to the Vantech dock doesn't hurt anything. Nor does it help. Before I found out that the update doesn't actually work, I decided to risk bricking one of my Vantecs to see if it could be updated too (having noticed that the binary firmware image did not mention A or B), and luckily it worked. Well, it didn't hurt anything. The GUID remains unchanged (just as for the NewerTech dock) at 0x30E002E0454647 and trying to attach multiple docks via Firewire still fails. Maybe if Oxford Semiconductor and NewerTech ever actually produce a fix that works, it can be applied to the Vantec units as well. I live in hope.
Update: See this post for the solution to the NewerTech problem, and, happily, it does work for the Vantec problem. If you don't want to risk upgrading the Vantec's 934DSA firmware with the 934DSB firmware provided by NewerTech, you could probably change the GUID alone, using the instructions in that post.