This is described in the game as:
Aids in the manipulation of the firearm.
This is found north-west of central Itacua province in Culta Village. Part of the first mission “Amaru’s Rescue” is nearby. The accessory case is on a shelf inside a burnt building in the middle of the village.
There’s a reasonable amount of enemies in the village, but if you can’t be bothered to sneak around taking them out quietly, there’s a super simple approach – take a helicopter and just hover in the village and let your teammates take everybody out. Your helicopter will take some damage, but this is Itacua… they won’t shoot you down.
The foregrip attachments (from this one up to the V4) all provide the same benefit and trade-off, just to varying degrees. Each improves handling while sacrificing a little accuracy. The Vertical Foregrip is the most basic, and provides the smallest improvement to handling over all the other foregrips available, with the least reduction to accuracy.
This is a nice accessory to try out on almost any gun, and if you like this then you can find the Foregrip V4 in Monte Puncu province and give that a go. Personally, I like to rock a foregrip on an LMG or shotgun for the added mobility.