Wasn't actually just made up by one of the instructors, it was in the manual of one of the major Moorabbin schools.
I am fully aware there is no published 'decision speed', but I imagine the reason for using that phrase is to suggest that if you haven't even gotten to Vyse when an engine fails, it's best just to treat it as a single failure and just get it back on the ground.
I would think that fewer would come to grief trying to get a twin landed and stopped before the fence (or go through it if necessary) than trying to get away on one engine before even achieving the best ROC speed. Especially if they are somewhat out of practice on asymmetric flying.