The Bren is a replacement for the Lewis not the Vickers. The Water cooled MG were concentrated in the MG Corps in 1916, later MG Bn at Brigade level totally different function. The Maxim ( which is shorthand for the slew of derivates - Vickers C is a 1912 design going into production in 1914. The M1917 is actually a 1900 design only adopted in 1917 after being rechambered for the 30.06 so not some super effective high tech fantasy gun. Its a water cooled MG that does what all do and the Russian Maxim is a 1910 design refined in the 30s and 40s.It’s also worth noting that Britain dug out single-shot martini-Henry carbines to equip the home guard, Moisin-nagants have also been on the front line in the Ukraine, and ammunition boots were British Army issue until the sixties. Just because armies have a lot of old crap tucked away in their warehouses or clogging up their supply chains does not immediately mean it is what they want to have, or want in those quantities.
Sure, if desperate it better than nothing but the Vickers-Maxim is a horrendous old Victorian fossil, and it being used by anyone after the twenties is a solid indicator of something having gone wrong somewhere. The M1917 is at least at the cutting edge of WW1 technology and designed for easy manufacture on early 20th C tooling, but it’s also not really the right answer to anything even before WW2.
Heck, even the British army loved the Vickers soooo much they bought the Vz26 (half a million or so), ZB-38 (40,000 or so) , hung spare Lewis & VGOs all over stuff, and used cheap Browning M1919s on many vehicles into the seventies.
The ZB 38 or BESA is a tank machine gun again as a replacement for the Vickers they actually bought 7 Vz26 and 30, and probably the same number of 53, all other production is in Britain or Commonwealth countries not bought from the Czechs any more than the SAW is bought from the Belgians. The Other air cooled MG are designed as aircraft MG and scavenged in small numbers, but then these guys are not above mounting Italian weapons either or ASW weapons on Matilda II for that matter.
The m1919 Brownings are not cheap they and the ammunition is free, lend lease, and every Sherman or Stuart left over from WW2 has a couple. By the 70s the standard calibre is 7.62 NATO and the MG is the GPMG. The one exception to that would be the Ferret which could be fitted with Bren GPMG or Browning. Others switch out the BESA or Browning leftovers for the GPMG over time.
But this is the 1930s. and