Cross-game party chat will be available to all PS4-owners, not just plus-members.
So if you're playing Uncharted 4 single-player, without plus, you could still form a party and chat with people on other games.
If the master-account on a PS4-system has plus-subscription, all sub-accounts using that PS4 will have access to the online play of PS-plus aswell - no need for family-subscriptions or similar.
Not certain if the sub-accounts get to shop at PS+ prices tough (10% extra on all sales), or download free games, but if the master-account downloads/buy them, they all can play them.
If you keep setting up the systems with the same master-account you shouldn't have trouble - but if one of the sub-acciounts sets registers a Vita with as a master-account, he'll not get access to the plus-stuff on the Vita, since the master-account there is not a PS-plus subscriber.
Then he will need to subscribe aswell.