I'm with you on this Don - I am looking to make a New Haven R-1-b Mountain myself, and finding a spare 6-axle Vanderbilt is difficult.
Lionel has all the parts to make a fairly accurate R-1-b for number series 3340-3348 (which are the ones that had Vanderbilt's assigned fairly often) - here's what I'm doing:
1. Base USRA Light Mountain (I'm using one from the original TMCC run in the early 2000s)
2. Swap out the USRA-style trailing truck for a Delta one (I have one on order from Lionel that I think should be a drop-in replacement)
3. Add the feedwater heater and piping to the top of the smokebox, and shielded air compressors to the pilot
4. Of course, the subject of this thread, a Vanderbilt tender. The tooling used on the C&O F-19 pacific from the early 2000s looks very close to the model of tender the New Haven used.
There are other details and specifics where series 3310-3339 would not be accurate using the USRA Mountain tooling, and 3300-3309 never had Vanderbilt tenders assigned.