I just upgraded my phone from a sony x10 (running z's rom/kernel) to a refurbished htc vivid for a penny (thru the att store, plus 25 bux tax+36 bux activation). refurb vivid is like new/clean display (the misses vivid has a very minor scratch on display)
the vivid is great, physically solid, and should likely come preloaded with ics 4.0.3 with sense 3.6. runs great as stock.
no problem unlocking with the help of htc, and no problem turning the partitioning security off (s-off). the s-off procedure (which i assume opens up the ability to flash kernel/recovery) does require you to void your phone's warranty by removing a "void" sticker on a screw to perform the s-off function.
DarkRaider seems to be a popular rom, and one that I'm running with no problems. it has a really nice gui installer once ran from recovery that works great with default settings, but has a lot of options for adding/removing app, features and replacing/using several different prepackaged kernel's on the fly, or even after installation.
I'm looking forward to dissecting the vivid roms in the near future; stock as well as custom and documenting them all on a file level.
the xperia was swell for me (upgraded from a moto backflip), but the vivid rules.
If anyone can suggest a fast and stable ROM for the Vivid, I would be most appreciative. I'd like it to be as close to ICS as possible, and, maybe this is the bad taste in my mouth from my Xperia, but I would also like it to have Sense 3.6.