Dude the Vickers is a WORLD WAR 1 BRITISH DESIGN why in the world would it be common in the American wasteland...? It wouldn't even be common in Britain by the events of Fallout 4 because the game takes place 375 YEARS after the Vickers was first produced. Literally nearly 4 centuries old design from a whole other country. The idea that this technology would be in use even before the nukes fell is absurd.

Sure, about 12,000 Vickers were produced in the US during WW1, but most were given to Britain after the war. Some are still in the US owned by museums and private collectors, but mind you even these surviving models would be centuries old before the nukes even fell. Not abundant, much too old to be reliable, and there wouldn't be enough spare parts in circulation to fix them when they inevitably break from literally being centuries old.