NetBSD/Vax 4.0 does indeed work. I managed to get it installed and running on a MicroVax 3900 earlier this year. The big problem with the distro is that the bootstrapping stuff does NOT work, and hasn't for a long time. I had to use the boot.mop from the 1.5.3 distro. Also, about the only way I found to install it is by netbooting. You can't install from tape. The documentation is horribly out of date, and attempting to follow the howtos is an exercise in frustration.
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4.0... Yes... But -current?
I'm running 4.99.73 on one of my VAXstation 4000/60s, and it has been compiling non-stop, running Apache, BIND 9.5, and other things, with no problems at all.
In NetBSD 4, the lack of shared library support made the system a bit problematic. In current, most things work pretty well - I can't find anything in the basic system which is broken with a few rare exceptions (ping6 dumping core with a floating exception when control-c is hit after just one packet is counted is one), but it's completely usable and all of the packages I've tried to compile and run so far, with the exception of php5, run just fine.
I'm very much looking forward to NetBSD 5 on VAX. John