What a nice little group!
That VCS/15 is a real rarity, only a few known, only b & e blocks so far. They are probably the lowest production maker of the consortium that year, possibly of all the Suhl makers. While Simson is assumed the lowest wartime maker, in 1915 they made at least 2200 rifles and the same can't be said of VCS in 1915. No way to tell how many they made as the consortium collaborated in production, but one thing is for sure, more Simson/15 are known and documented than VCS or CGH from 1915 (more JPS/15 are known though, probably the highest maker from Suhl in 1915 because they number more than CGH & VCS combined in 1915).
I would bet Simson made rifles are more common than any of the consortium makers individually, though collectively they probably made more. Storz figures would probably tell from a contractual perspective, but going by survival rates I think this holds true. (Simson rifles are more common than any of the others individually, but collectively they made more than Simson, and as they made rifles collectively they probably should be considered one makers from a production perspective.)