However, if I don't provoke these errors by logging into my laptop with ssh, in general use I now get quite an unstable connection with disassociation after a minute or so of use, so there is also something else amiss with the latest git version.
On examining this further, I have had your kernel running all morning on my laptop and the disassociation is not very problematic after all. The association is dropped at 30 minutes intervals and then it reassociates itself again without any dramas - probably this is something done by the AP rather than rt2x00. Apart from the vendor_request_error lock-ups when I try to ssh into my laptop, it is working quite well.
I am going to go back to a stable kernel + legacy driver again now but if there is any further information you need about the vendor_request_error lock-ups, let me know.