Thank you, I may have overreacted a bit at the mention of Facebook, my apologies.
I guess we agree that this must have been in 1972 then, when Air Support Command (not Transport Command as Wiki incorrectly states) was incorporated into Strike Command. I'm still wondering whether there were more instances where a Vulcan and VC10 flew in formation, and whether it would be possible to know more about the Vulcan involved (probably not). Two VC10s (XR807 and XR808) flew in formation with XH558 in 2009 but until I found the photo above, I thought that that must have been a unique event, so it's nice to have this period image to disprove my assumption.