Not necessarily the case; these days, it is a hard-nosed commercial basis through & through, with less of the "passionate" pilots. A lot are doing instructing as a stepping stone to hours building (as they spent £££ to get a "frozen CPL) on the way to a better job.
You also have to consider the links between RAF/cadets/VGS/AEF. Some are very back & white, such as a lot of AEF pilots are volunteers = little or salary to be paid. Other links are a bit more tenuous, such as having the relevant AEF/VGS operating on a military base rather than a civilian airport = additional exposure to the RAF. What about the numbers game - would a civilian flying club be able to have 6-8 aircraft available all day (on a regular basis) for cadet activities? I doubt it very much, as even with a contract, the staff/infra-structure/maintenance requirement would be quite unwieldy.
Oh for the good old days when an AEF would potter down to summer camp & operate locally for a couple of days!
Showing my age, cadet gliding was always winch-launched, none of these hybrid beasties with a whirly thing on the front! Would it it better to move towards "proper" gliders only? Much less maintenance & associated costs - but on the down side, perhaps less airborne time than in a Vigilant? For a typical winch-launched glider, you might get 4 launches/landings in the time that a Vigilante would be airborne for 30 mins? However, that's 4 take-offs/landing for a cadet to have a go at (if suitable!). Then make advanced gliding towed for greater duration.