Well, I've been back and forth on this issue, and solicited feed back from the Mudcat forum ... had lots of good advice pro and con, but I went ahead and installed a fishman banjo pick up yesterday, in my Vega.
I have been struggling with the lack of control, and occassionally presence, with the banjo mix. The band plays a variety of venues, and we often mix the sound from the stage - mics are the enemy! Sometimes, the sound is excellent with a mic - but too often it's just hard to tell ... so I read up on what Deering had to say (great comments on banjo pickups here, by the way - on their FAQs page). For a little more then $100 I bought the Fishman banjo pick up and installed easily myself yesterday. I played with it through an amp in the house and it sounded pretty true - not surprising; Fishman makes great guitar pick ups. I have not used a pre amp with yet (appears to be reccomended, though I did get a fairly strong signal from it), nor have I run it through the mixer but I'll get that chance this weekend. I'll come back to this thread with feedback on how it goes.
ANYWAY - the pick up itself is pretty interesting. It uses magnetic coils, like an electric guitar. It is mounted beneath the head, just north of the bridge inside the sound ring, on the truss rods - attached to the backside of the head, you tape a free standing steel plate (very thin, like tin foil, and perhaps 1/4" square). That plate vibrates with the head and moves inside the magnetic field created by the coiled pick up - inducing the electronic signal that coresponds to sound. It is a bit like combining the two different technologies of piezo eletronics (ie., most acoutic pick ups use sound pressure to pick up the signal from the instruments wood) and electro magenetics used by electric guitars (ie., an inductor - the string - moving inside a magnetic field induces a current representing the sound). Pretty ingenious, and very promising! I am hopeful that it will work well on stage.
Now; the same old problem of pick ups in my guitar; I am now playing an electric, and it is a bit of a different instrument!