Dicko
I have a mate that is a chef, adn he says ya really need heat of some sort to get the beans to release the flavours. I recently did this with a 23L batch of porter, I put 1.5 beans (split them and scrape them - the tacky black tar in the middle is actually where the flavour/aroma comes from, they are TINY black seeds, the bean itself is just a pod) in a pan with about 150mL of water, and brought it to a boil, simmered for 5 mins, then added that the a fermenter, and racked ontop of them.
The vanilla flavour is perceptible, but hard to really pinpoint as "vanilla", just adds a certain "something" that you recognise, but cant pick, which, IMO, is the hallmark of good spicing. However, in 51L of cream ale, I would go for at least 4 whole beans, maybe even 6 if you want it to be stronger in vanilla.
I bought my packet from Coles, IIRC, and it is MasterFoods vanilla beans (7g). It cost $9.90 or something like that, and there were 4 or 5 whole beans in there. If I were in your shoes, I would spend the $10 as an experiment, cause over the course of each bottle/glass, it will really be very little price, and it will be adding the effect that you are after. I, personally, will next time go for 2, or 2.5 beans, but as a starting point, I am very happy I went with less, rather than more. I would say take a sample after a week, and if it doesnt have enough vanilla character, then just add another bean or 2.
All the best, and hope that helps a little
Trent
EDIT - As Duff mentioned, they can be a bit pricey for a brew, but generally I am fairly happy to spend whatever to get the beer I envisage (anyone know how to buy brewing talent? :lol
My mate also mentioned that if you can get them at a catering type place, that sells foods to restaurants, the beans will generally be fresher. I think you will find that the older the beans get, the more brittle they are, and the fresher, the more bendy, and black/tarry. If you buy them from a restaurant supply place, they will work out cheaper per bean, but you may need to buy 20 at a time or something like that. Please let us know how it turns out, and if it was great, how you did it. Actually, if it turns out amazing, just PM me, so I can get an advantage in the next brew comp :lol:. Just kidding, BTW.
T.