Are you waiting until you backsweeten before adding vanilla extract? I expect the sugar will alter the taste perception, so waiting until then is probably a good idea.I have a 1 gallon and a 1/2 gallon batch to play with. I'm thinking possibly a tsp per gallon but adding it in increments. We'll see, that's months away.
Are you using commercial vanilla extract or home made? Using the stuff I make, I'd add 1/2 tsp to the batch, give it a week to integrate, then taste test. [My current batch steeped for 4.5 years -- I didn't need it and forgot about it under the sink.]