There was a tendency for NBC companies to purchase the Bristol/ECW combination, and while I'm not entirely au fait with NBC practice, I believe this stemmed from the fact that the former Tilling companies were obliged to purchase in-house. Wasn't there also a period where Bristol and ECW could only sell to nationalised concerns - hence the development of the Dennis Loline, which was essentially a Bristol FLF built under licence?
The VRT was a bus I'd never encountered until Stagecoach took over my local company (Northern Scottish) in the early 90s and they sent a load north in exchange for newer Olympians.
A couple of SBG companies had new VRTs in the early days, but they didn't like them and swapped them for FLFs. I can't figure out why - as a mere travelling passenger the VRT was no worse than the Fleetline which was their preferred rear-engined bus!
The BCV/ECW combo was indeed restricted to the Tilling Group though this was later relaxed before the NBC was created (e.g. South Shields Corporation received some early RE/ECWs).
The VRL and VRT were built in a bit of a hurry with some last minute changes to the design and a lack of evaluation and testing in order to try and catch up with the OMO designs of the Atlantean and the Fleetline. That is apparent in some of the weaknesses that are apparent (e.g. the transverse drive train into the rear axle etc). The VRL had more drivetrain issues and was eventually killed off in favour of the VRT with many of the issues being resolved with the Mk2.
The SBG received many early examples in their aim to get OMO deckers from their preferred supplier. However, the design flaws were one thing but also the SBG were very conservative in their thinking so weren't really geared up for something as advanced - remember they were still buying crash gearbox Leopards in the late 1970's!! Therefore, they couldn't keep them on the road so in 1973, there was a swap with the VRs heading to England (most to NBC) with late model FLF Lodekkas heading north. NOTE: Central SMT also had a batch of Fleetlines (J reg?) and they didn't get on with those either!
The English operators found that there was actually relatively little wrong with the VRTs and they worked soundly for the next 10 years with some of them having long lives (e.g. some Eastern Scottish ones went to Eastern National and ended up with Ribble for post de-reg services).
The English FLFs were much more to SBG's liking, working mainly for Eastern Scottish and Western SMT but, of course, had to be crew operated. Eventually, SBG settled on Fleetlines and the locally built Volvo Ailsa (front engined) for their deckers from the mid 1970s onwards with Eastern Scottish (the most English of SBG firms) getting Fleetlines with ECW bodies!
Oddly, Tayside bought a batch of VRs (1976?) but oddly didn't get on with them so sold them after about 6 years service to Burnley and National Welsh among others.
Eventually, deregulation and the fall out of that saw VRs heading north of the border with various independents buying them for competitive services and schools (e.g. Moffat and Williamson, Gaelicbus) and also former SBG firms such as Lowland and then via the Stagecoach empire to Fife and Bluebird. Some passed via acquisitions to Kelvin Central though don't know if any survived to First. Hope this helps