It’s an interesting question, all the more so because the Vulcan was very rarely flown for endurance rather than range. My feeling is that had a sortie been planned for maximum endurance; i.e. minimum fuel flow; then comfortably in excess of 7 hours would have been achievable. Unless someone has a copy of the ODM lurking in their loft (and any of us can remember how to use it!) then it will be difficult to confirm.
The Nimrod OTOH would probably be flown for endurance for at least a portion of every sortie, although not for maximum endurance but to obtain the lowest fuel flow at the altitude dictated by tactical considerations. This might entail shutting down an engine(s); 2+1+1 or 2+2; but if maximum endurance was sought without constraint then it would be all 4 at optimum altitude, as high as possible. On one sortie when maximum endurance was the objective I went over 10 hours with a high Zero Fuel Weight aircraft.
Those who went on to fly rather more modern machinery will be well aware of the effect of entering Cost Index 0 in the FMCG and watching the managed speed drop to an alarmingly low value! You could save a lot of fuel but spend all day getting there.
YS