Your passwords wouldn't be very secure if they were stored in plain text in a config file, would they?
They're in your user password store, which is probably called "Passwords and Keys". Open Passwords and Keys, select "Login" in the nav bar on the left, and look for entries like this:
VPN password secret for PIA - US Silicon Valley/org.freedesktop.NetworkManager.openvpn/vpn. I'm not aware of any way to do bulk entry, but you could enter your passwords by hand there.
What I do is just wait until I use a particular server and then enter the password at login. It's kind of a hassle, but your system should remember the password so you won't have to enter it again for that server.