I successfully used $400 in UA Type B VDB vouchers to ticket an itinerary with all CO-operated segments at SAT today. Booked and held the reservation over the phone with a UA representative earlier this morning. Although all the flight segments are operated by CO, I deliberately build the itinerary with UA flight numbers. Called UA twice; the first agent said it wouldn't work (so I politely disengaged and hung up); the second agent said it would. Headed over to the airport ticket counter this afternoon, gave the agent manning the ticket counter the record locator number, the Type B vouchers, and my credit card to take care of the small residual balance, and the agent manning the ticket counter applied the Type B vouchers and ticketed the reservation with no problems and no fuss.
My advice for anyone who is looking to do the same would be two-fold: first, try to build the reservation with UA flight numbers only; and second, keep trying until you find an agent willing to accept the vouchers in ticketing it.