Actually no, the Water Cooled .30 cal M1917 is not less common than the M1919 .30 cal (Air cooled), the M1917 was used for heavy positions (MG nests), whereas M1919 was used in squads as a MMG. The reason we see it more is because it's lighter than the M1917 and so a man can carry it with less problems, whereas the M1917 is a bit heavier and unwieldy to carry.
And the Vickers is just the licensed version of Hiram Maxims Machine Gun, as the original gun was built in .303 for British trials IIRC. Vickers didn't modify too much on it.