Tim,
On the keypublishing link there was a suggestion that the Vulcan's had difficulty getting off the ground in Ascension.
From my experience in Aden and Butterworth a fully armed and fuelled Vulcan on unrestricted power had a ground roll of about 4500 feet. In contrast, at Butterworth a Victor 1 had a ground roll of about 8050 feet. The runway was slightly shorter than this
As for Black Buck, from the horses mouth so to speak:
"Apart from a failure to pressurise I do not remember any problems getting a Vulcan airborne., I expect the Victors would have appreciated RATOG more."