Weekend flying will be VOC's bread and butter, the sooner MA work this out the better and have a process in place for a engineer to be on call during the weekends during air show season.
If VOC require that level of service then they will have to stipulate it in the support contract and pay for it. If they have already done so then no doubt MA will cover any losses incurred by the non-appearance of the aircraft due to their inability to fulfil a contractual obligation. If they haven't done so then it's all down to them.
I don't know how many MA engineers are endorsed as certifying engineers for the Vulcan. If I were to guess I would put the number in the low single figures; maybe someone out there knows the actual figure. These people will also have "day jobs" as well, so given the numbers and the other committments it might not come as a surprise that no one is available.
It really is very simple, VOC is a commercial operation and they will get the support level they pay for.
YS