I think I understand what you're suggesting, but how does that differ from using my paid VPN service from my mobile devices when I'm away from the house? The issue is having my mobile devices auto-sense my home network (or any other trusted network) and deactivate their built in VPN clients. There's one MacOS/iOS specific VPN service I've found (www.getcloak.com) where the client can auto-detect trusted networks, but it's quite expensive and seems to be aimed at small businesses rather than home users.
It's a shame iOS can identify a trusted network and enable/disable VPN, if configured, accordingly.
Most VPNs only allow one connection per subscriber, which is why you should use a single VPN service from your home router. Of course, if you feel like duplicating your VPN subscriptions, then by all means have a VPN for each and every device.