In most cases a VPN is redundant to a seedbox, they are two competing ways to accomplish the same thing, concealing your home IP address.
Using both to conceal your home address, is like using both a belt and braces. Redundant.
In the pursuit of sub rosa traffic, the advantage goes to seedboxes, to use a VPN, your home machine is an active part of your network activity. With a seedbox, you can turn off your home computer and still be seeding. Additionally, VPNs are known to leaks and have other problems, like WebRTC, DNS issues and suffer from congestion. Your home address can't be traced if you are not there.
So what are the advantages of running a VPN?
A VPN from your SB provider allows you from home, to take advantage of that fact, your network activity can appear to be originate from the IP address of your seedbox. Not exposing your home address to where every you go. Much like a proxy. So web browsing, ie signing up for a tracker, can appear to come from your seedbox.
3rd party service.There are two advantages of using a seedbox with a 3rd party service:
One, 3rd party services often allow you to designate where you terminate, this can allow you to bypass geolocking.
Two, you can conceal your IP address from your Seedbox vendor, though at a usually a significant hit in download performance.
The argument that a 3rd part VPN allows you to conceal your network traffic from your ISP, the same can be accomplished using common encrypted protocols, https, ftps, sftp, only thing visible is at the endpoints. Additionally it is unlikely that your ISP is paying any attention to your network traffic (ISPs avoid traffic content monitoring for reasons of liability)
