greymount,
i'm planning a build of the exact same rifle!, have my stock blank and plan to order parts within the next few days.
i have spent allot of time researching what information there is to be had on the net....there's not too much about vincents as compared to other, earlier makers, to be seen.
as for changing the architecture by moving the barrel a 1/4 inch or so, we can only work with what we are able to obtain as far as parts and i don't think the architecture would be degraded if you have to move the barrel a bit to accomidate the available parts.
i wouldn't put the drum into the plug for the sake of deviating that little bit. there are many threads/post on this and other sites that discuss the problems associated with the drumm threads invading the plug threads, most of them are centered around gas leakage out the barrel/plug threads and the eventual repair. avoid this by keeping the drum at least 1/8" ahead to the end of the plug. the eventual, almost inevetable need to rebreach the barrel isn't worth sticking that closely to his architecture. besides, unless an original is readily avaiable for comparison, the end of the barrel location won't be that readily noticed, at least that is my thinking. i plan to do just exactly what i just said, for what that's worth.
the problem you have is common to allot of builds because i think people take for granted that when a stock is pre-inlet/ carved, everything is suposed to go right where the morticing locates it, that's not always true or possible, for that matter. if you talk to any of the more experienced builders they will all tell you that just because it is a pre- this or that stock, allot of tweeking still has to be done. the stock patterns are made to accomodate a wide range of parts, so they can't produce a stock that is machined too close to finished condition for any one particular set of parts.
tell me a little about your build, is it TOW,s kit or Muzzleloader builders' supply's. other than the available books did you have or find a good source of information about his guns on the net?
what length and diameter a barrel are you using. i would like to use more than 36 inches on mine as most of his guns that i have seen or read about are in the 39 to 41 inch range. i was thinking of getting a 42 inch 7/8 dia. barrel and cutting it to the length i decide on.
in that i am working from a blank would you tell me, how long is the barrel channel on your stock the dimension from drum center to end of forearm would be great!.
good luck with your build, and maybe we can keep in touch and guide each other along by comparing notes as we go.