To me, "equal vulnerability" means both sides vulnerable or both not.
In sacrificing against a major suit game, "down two" means -500 when vulnerable versus -300 when not. This compares against opposing games (plus bonuses) of 620 and 420 respectively. In either case, you are saving 120 points.
But if you go down one (undoubled) against a part score, it's -50 not vulnerable, and -100 vulnerable. Counting a part score bonus of 50, either result represents a savings against a part score of 60+50=110. But down one vulnerable represents a loss against a part score of 40 (plus the 50 bonus for 90).
So the two "equal vulnerabilities" are "equal" against a game but not against all part scores. Is this a large enough factor to bid differently when both vulnerable than both not? This question is probably more applicable at matchpoints and perhaps IMPS than rubber.