There is an app. I think it's Launch At Startup

Macrodroid can help you achieve this, I use it to automatically sign into the UK BT WiFi network & also to launch ad free modded Spotify and smart YouTube next (so I can cast ad free from my phone to another phone or android box) you can see an example set up here to get an idea, add trigger (device boot) and actions would be load app, and then can use UI interaction to click the connect button :)

See how it looks and works with my main usage :)

https://github.com/aidanmacgregor/BT_Wi-Fi_Autologin_MACRODROID-WISPr-HTTP_POST-HTTP_GET-OpenWRT

Use "Launch On Boot" app: https://forum.xda-developers.com/t/guide-tcl-autostart-autolaunch-app-with-launch-on-boot.4450711/

Don't most VPN apps have an autoconnect option. Every one I've used does.

you can setup vpn on your router.