I'm not familiar with Banana mags and their pouches, but since they are technically RPK mags I'd think they'd have special kit anyway.

They wouldn't fit in any modern rig designed for AK or Stanag size magazines, nor would they fit in a pouch for LMG boxes. Best you could do is carry them in a dump pouch but that would get annoying quickly (irl).

That said, vests in the game seem to be a design decision and balancing factor. The first few, which are standard looking chest rigs/plate carrier designs, carry 4 magazines. I carry 4 magazines in my battle belt daily. I don't know which direction they'll go, I'd prefer ultra realism myself but I also prefer some hard limitations. Recoil, for example, in real life it doesn't exist for almost every rifle in Tarkov at the moment - even in full auto. It would be cool to see this reflected in a game for once, but it would also be cancer because if it were ultra realistic, you'd only shoot on full auto in your hideout/bunker shooting range.