Looks like it was sold in the US market by the Merimont Watch Company and also signed by Vacheron. The movement number matches the Certificate.
The Merimont logo on the bridge is typical for that company. They also made movements or sold them in watches not signed by Cacheron, and the movement design is very similar to that of VC. They probably had some marketing/financial arrangement with Vacheron.
Here is a comment by dean from 2007:
"In terms of finish and construction, they are equal to V&C of the period. In fact, there are a few movements co-branded Merimont and Vacheron & Constantin. This name was only used in the American market and may have been for importation purposes, as suggested by it's seperate incorporation as the Merimont Watch Co. "
It's a real gem, Bracky.
The dial needs a little work but the movement looks in decent shape (nice perlage).
Very nice font and hands on the dial.
You should have it restored but I think without actually seeing the watch, it may not need that externsive work. It's hard to tell.
You could have the VC Boutique in New York have a look at it for starters.
Keep us posted. Your great uncle had wonderful taste.