In which case, we're back in the territory of special effect attacks.
But wouldn't it be a wash anyway?
Let's talk through it, mechanically.
On subsequent exchanges, if the emotion is still in place you may continue to feed, gaining a +1 on the roll
If the target has to have a viable emotion in order for you to feed on them... the way I'd run that is that I'd require the WCVamp to either do a maneuver, or make a declaration/assessment to place an emotion aspect on the target.
They then use that aspect not to gain a bonus to their roll... but to be able to feed in the first place.
If it's a fragile aspect, then using it to gain a +2 would make it go away... at which point you wouldn't be able to feed off them anymore.
If it's sticky, then using it to gain a +2 would still be problematic (easier to resist, since you have to split your shifts between power and duration), repeated uses cost FP, and it still eventually goes away, at which point you can't feed anymore.
So if you're concerned about the WCVamp who gets a free maneuver with his attack... I'm concerned about the WCVamp who's dumb enough to cure his target mid-feeding.
Sure, they can re-establish the emotion next round, with another combined action... but I'd say that resets the process, too. They'd be trading their +1 for subsequent exchanges for a one-time +2 each time... not very broken all told, IMO.