Yeah this seems to be a new issue where reddit is partially blocking VPNs now as I also have to use old.reddit.
have you considered self-hosting your own DNS server? Gives you a lot more granular control over what you are blocking.
This is very much dependent on your use case. VPNs and DNS do different things.
If you use a VPN all the time, some people just recommend using that VPN providers DNS (not sure what PGs stance on this is) instead of changing it but, you could also run whatever DNS and whatever VPN you want as well if you decided to configure things that way.