Probably best done with a check-box in the provider setup dialog and then an ip-up script in the generated OpenVPN config file. I don't use any VPN addons myself so that's just a suggestion, there are a number of ways it could be done.
I don't see why an addon would throw away your autostart.sh, that would make a lot of people angry. It should be fine to use that. You can use tun+ instead of tun0 if you happen to have more than one VPN at a time as well. Yes, systemd is another way of running scripts during start.