Using a VPN won't give you much extra privacy unless you don't trust your instance's admins. IP addresses aren't federated, only the content. I don't think Lemmy itself stores any IP addresses, they merely show up in NGINX logs which ideally are rotated fairly frequently anyway.
But at that point if you care that much, you should already be using a VPN 24/7, because practically everything you're logged into has your account tied to an IP at some point and any of them are potentially vulnerable to a breach.
I'm just careful about what I post and what I vote on, this identity is fairly public anyway.