Following recent discussions this is the information I gathered regarding VXN stamped movements
scan courtesy of Tic-Talk
Due to the impossibility of exporting gold cases to the US for tax and custom reasons from circa 1935-1950 Vacheron Constantin movements (assembled) were sent directly to the importer who then had them cased in gold. The rule for the case back inscriptions are rather shady as some were inscribed Vacheron & Constantin, Vacheron & Constantin New York or just with a Maltese Cross.
All movements were however stamped VXN.