VCC or MF crew should not be used if legacy crew are sat at home in my opinion. VCC should be doing whatever they were recruited to do in the first place. We have enough crew to cover the operation and other people should not be taking our jobs.
Whilst I agree with your opinion unfortunately there is no mention in the DA about the use or not of VCC's, MF or temps.
Having said that BA were right to use long haul aircraft and MF crew on European routes to ensure as many of our passengers got home for Christmas. Legacy crew just do not offer enough flexibility even with the DA in force to operate these types of services.
It is due to this lack of flexibility that has forced BA to introduce MF and is caused purely by Bassa's refusal to negotiate over the last ten years or so.