Since the update 0.9.6 is impossible to access to the attachment inventory on equipment, now you only access to the items that are inside the attachments or the vest/equipment himself. Not sure if this is done for some reason or is a bug, but can give the idea of have another keybind to access to that inventory where you can remove or add pouches or parts of equipment into a vest for example. Limit this feature to only add things and can't remove any only restrict a cool mechanic of full customization in game.