VPN's DO NOT protect you from serialized pixel trackers. This is because a separate image URL is made for each recipient. It's not about the IP address at all. It's simply that each image generated (on its own URL) is sent to only a single recipient. If the serialized image URL gets accessed, then they know EXACTLY which email address has viewed their image.
SIMPLE.
One very good way around this which some better privacy-protecting email services use is to access EVERY image in every email and. cache it (create a copy). When they do this, they render the technique effectively useless, since every image gets accessed. Of course, most of the spammers don't care because the cost of sending these emails is practically nil.
An even smarter email service could implement a method even a little better: Set up honeypot accounts and which are widely registered to mailing lists so that they receive a lot of spam. As messages are received, scan and recognize the image URLs, and , and then load images from ONLY those which appear to be pixel trackers. There still are limitations to this method, but yet it's still not all that much trouble for a provider to implement considering all of the good it can do.
Unfortunately, this whole situation is an endless process of rivals making measures and countermeasures.