Petet,
I have no knowledge regarding Vulcan ops but the 28lb bomb was used as a practise bomb on many aircraft types (I dropped them from Hunter and Buccaneer). It aimed to simulate the ballistic characteristics of a 1000 lb bomb fitted with a 114 tail (free fall delivery/low drag). There were variants that produced 'smoke and flash' to aid spotting on ranges and there were inert variants that were dropped on some ranges if a potential fire or EOD hazard existed.