VPNs don't prevent tracking, especially when you're logging into services.
They can help obfuscate your identity to varying degrees, but honestly this is a pretty odd decision. I'm guessing it has more to do with malicious activity, or some other type of activities that Reddit is trying to curtail, and they feel blocking VPN IP ranges will help them.
Sometimes I read reddit post while not logged in on old.reddit with vpn on. Just the other day I got this message. After changing servers the message went away
Many users don't log in. After the fall of the 3rd party apps, I only use the rdx webapp if I want to view reddit. I don't log in with an account at all anymore. So all of my data looks like some anon browsing from the same VPN server as hundreds of other anons. Yes they can analyze all of that traffic and individualize it, but that takes work. I'm glad to make them expend more effort, even if they get the same data in the end. Every step that makes it less cost effective for them is better, even if not perfect.