The purpose of the hi-viz vest is derived from busy civvie dispersals where vehicles converge on aircraft from all directions. Although there are dedicated "roads" marked around the dispersals, vehicles do need to leave those lanes to get to the jets. Hence the wise safety rule at civvie airports for hi-viz vests.
Having been to Brize and Lyneham, many times, where MT criss-crosses the dispersals, it would be a good idea to make it mandatory at those airfields for personnel to wear hi-viz vests when not in the immediate vicinity of an aircraft (ie safe from properly driven MT). A rule requiring us to wear a hi-viz vest at all times and in all places on all RAF airfields is flawed.
However, in reply to the guy who says that he will do his own thing: wrong. You do as you're told, even if the rule is flawed.