being told that the seats were the same as in Valiants
Not quite. Tha Valiant had the Mk4 seat that had three cartridges and left an eight foot telescopic gun at 80ft/sec so that the pilot would clear the tailplane. It was rated at 90 kts/0ft although the canopy would not jettison cleanly below 120 knots.
The Vampire's was a single cartridge job that was rated at 120 kts/200ft.
I went on a trip in 1961 shooting GCAs at Sleap airfield. Looking up at break off there was just a load of grass because there was about five GCA approaches and not all of them used the runways.