In Vietnam and into the 90's M16 pistol grips were sometimes repurposed as vertical grips, sometimes chopped and sometimes not, and just bolted onto carbine handguards. You see pics of XM177s with them now and again. That's what I did with my GUU clone above, that's a chopped A1 grip I'm using as a VFG there.
As far as purpose built vertical grips, I'm not really sure who came up with the first one to hit the market, Knights maybe? But everyone jumped on that bandwagon pretty quickly.
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