It's an often told story but I have not seen actual sources. Vodka had been made to "polugar" ("полугар",literally "half-burn") measure long before Mendeleev has been born. "Polugar" was a spirit, which could be burned down to half of the volume (when heated almost to boiling, of course) and it's about 38% ABV. More plausible theory is that when the better methods of measuring alcohol contents became available the standard had been set up to the "round" 40%.
I wouldn't say it is a stereotype, it has practical reasons - 40% mixture is the max that body can consume right away. More than that and spirit stays in blood so it just enough to drink pure water next day and you are drunk again. Tried couple of times - not that pleasing effect.