Finally had a chance to sit down with my morning coffee and flip through the archive photos in Carole Lambelet's book The World of Vacheron Constantin.
It was interesting to realize that, as these were the early days of wrist watch design, the styles were very much of the period. Therefore, certain features were indiciative of particular years. I'm sure those working in the Heritage Department can date a watch quite accurately from style alone!
The plain cushion case on bracelet with round dial started to appear in 1919, in both men's and ladies watches. Cathedral hands were a feature on pocket watches from 1920. The Milanese-style bracelet appeared on men's cushion cases in 1922. The distinctive arched Vacheron & Constantin name was applied to small cases starting in 1927. Finally, the closest representation of your watch, on strap not bracelet, appears as photo reference #2888, in 1930, with case number 257883 on the featured piece.
I believe your dial is redone and the "blob" at 6 o'clock should be an applied numeral. Now, if you please, post some pics of the movement and case back