UPDATE:
FWIW, I found some information buried on the Tektronix webpage regarding this issue:
http://www.tek.com/support/faqs/what...nd-vectorscope
The older units use a bulb type OL-381 or BPR-022. This is a VERY strange little bulb, with 2 recessed socket contacts on the base. The "socket" soldered onto the PCB is just 2 little pins that the bulb plugs onto.
I found a couple places that could get these bulbs, but prices ranged from 8-10 dollars EACH.
Not wanting to spend $40 for 4 tiny bulbs, I retrofitted the unit for type 7381 bulbs (same voltage/current, but readily available and about 40 cents apiece). I replaced the original sockets with contacts removed from right angle DIP sockets, to provide a place to plug the new bulbs in. Everything works great, and future bulb changes will be a lot cheaper.
Still would love to have a copy of the service manual/schematics for these scopes if anyone has a lead on them.