[quoteWhy didn't they just buy the standard off the shelf, certified version ][/quote]
When bought in 1990 the Vigilant were just that, Grob 109b just like anyone else could buy. But then the fuel system was altered (improved?) to improve hot starting, the instrument panel layout was altered, Mag and engine oil experiments reduced the number engine misfires and dayglo patches were stuck on the wings. However the biggest change from what Grob produced was the change to the undercarriage legs as Grobs design kept snapping when used by the Air Cadets on grass runways (never the tarmac strips, odd that). The new legs were designed by the boffins at Farnborough and reduced the number of new propellors needed by not breaking.