It's kinda complicated while being connected with OpenVPN!
Port forwarding on your router won't work, as any traffic in would get forwarded to the box, but the box will be replying over the VPN (so from a different IP address) which will get blocked, it would be like saying "Hello" to someone and someone else answering, you would probably just ignore them
Connecting you laptop to the PIA VPN won't work either, just because both are on the same VPN doesn't mean you will have access to your box or other computers on the VPN. (Something like hamachi could do this, but I don't think any VPN providers give this option)
Some VPN providers allow port-forwarding, I think PIA allow it too but on a dynamic port, your best option would be to ask on their forums.
I use AirVPN and they allow port forwarding of any ports above 2048, so for that to work you would need to change the port for Openwebif,
although I'm not sure how secure it is having ports forwarded directly to your box.