Yes, 20% is most likely too high.
FYI, I normally use around 5%(revised) for TFA Blueberry Wild & then you can add 1-3% Vanilla to that also.
If you've got the TFA Ripe Banana, start around 2-3% and work up as it's kinda strong IMO.
EDIT: If the Vanilla you have is TFA Vanilla Swirl, it's good one.
Makes fruits have a more Creamy/Vanilla effect, and less sharp/harsh.
I also like to use 1-2% Sweet Cream with it, TFA or CAP.
Or Hangsen Cream at .5% (point 5).
You can add these to the Banana too, for a Banana Cream thingy.