Vegemite is developed by Australian, as big as Coca Cola to US. It is NOT REAL YEAST for rising bread/馒头/etc.
Vegemite is well know in the vegetarian world, developed from Brewer's Yeast - the richest known natural sources of the vitamin B group. (Do not use Brewer's Yeast to rise bread) For more info, search web.
If u still have your 发不起来的大面团, try adding regular yeast to get it going again. Happy cooking!