Vanilla makes everything better!
By "fresh" beans I hope you mean that they've already been killed and are brown but moist. I've looked at growing vanilla but outside of the tropics it doesn't seem cost-effective.
Anywhere from 0.5 to 2 or more beans per gallon will impart vanilla flavor.
The best approach imho is the following:
On brew day, split and chop the bean(s), and put them in a shot glass or some small glass container. Add just enough bourbon (or rum is fine) to submerge the beans. Cover and leave them sit for 1-2 weeks while the beer ferments. Add the mixture either to secondary for a week or so, or strain and add at bottling/kegging to taste.
The vanilla flavor will fade over time.
Cheers