An elusive man, Mr Vanetzian. There's a marriage record for him in London in 1942, but otherwise I'm coming up empty apart for some patents for model railway tracks. Vanetzian is certainly an Armenian name and - circumstantially - Alexandre/Alexander may be related to (son of?) an Armenian caviar exporter who turns up in various sources as either Gregoire or Gregory in the 1920s. Possibly operating from or via France at that time; I did find a 1925 reference to a 'M Vanetzian' (as in Monsieur), who was the son of Gregoire/Gregory, but whether that's your man is impossible to tell. It probably isn't, though; the family caviar company was called Grivan Products (still trading under a different name, but I doubt there's any Vanetzian involvement now) and the British end was run from Soho by Peter Vanetzian, who joined the firm in 1921. Again circumstantially, he might have been Alexandre/Alexander's brother.
It would be interesting to know the original source for Mr Vanetzian's supposed Mercedes Benz connections, which gets repeated in quite a few places. Lines Bros publicity? Meccano Magazine? Railway Modeller? Certainly not a works driver, so probably a casual statement like "I once had a Mercedes, which I used to race sometimes" has become "raced for Mercedes Benz".
Purely a guess, but my money would be on a bit of minor European rallying or some sort of event in one or more of those 'automobile weeks' in the inter-war period.