I went with the straight 2-shifts per refresh that it suggests (with the toughness bypass as a bit extra), but you could certainly modify this. Yes, it's very situational, which makes it difficult to judge the refresh vs effect balance. The problem is that the effect is pretty significant, too, when it comes into play. Consider that even the toughest vampires with max physical stress boxes and Mythic Toughness would take a mild consequence, and less tough vampires (which should be most of them) would get hit with a moderate. Oh, you might change 'toughness does not reduce' to 'counts as The Catch' if you want to beef it up a bit; this would mean that Recovery can't help reduce the consequence's duration.
Other options might include:
* Make the attack 6-stress plus one per stress inflicted on the character being bit
* Allow the character being bit to spend a Fate to boost the attack strength by +2
* Expand the usefulness by treating the character's blood kind of like holy water; ie, it works on touch and can be used as an attack
Hard to say, though, what would end up being a good balance. I think the last would be too much, for example.