It was a standard practice on the Trident to deploy the reversers in flight, But I can't say if it was on a VC10.
I'm remember there was no physical interlock (Wow switches) to stop you doing so on the VC10, but I don't know if they ever did or not in flight. The system was purely mechanical. There was a light that came on on the dashboard to indicate the buckets were unlocked but it was just that a light.
All RAF VC10's only had two reversers fitted where as the early BOAC tens had all four reversers fitted.
To deploy the reversers the throttles had to be pulled all the way back to Idle to release the reverse handle which were on the front of the two centre throttle levers. As you lifted those handles they deployed the buckets. Then as you pulled the front handle further up and over towards you that then increased engine RPM by rotating the actual throttle valve at the engine end in the reverse sense, it only being closed in the mid position.
That's as I remember the system, but it was 25 years ago I was last in the cockpit.
I hope that helps.