I'm sure you know PN, but the VC-10 was considered as a launch platform for Skybolt, as were several other types at the time according to 'Vulcan's Hammer' by Chris Gibson.
The Vulcan B3 was meant as an Airborne Alert aircraft, carrying Skybolt too. Pity that one never got of the drawing board. Upgraded engines with reheat. Bigger crew compartment (spare crew as well) all in ejection seats.
Haven't read the book for a while but pretty sure this is all correct.