Found it! For anyone in the same boat, just go to the official website and use the "Lost License Key" retrieval. You just need your email. Don't bother with keygens – those are full of malware. VueScan is cheap enough for a lifetime license anyway. Option 2: The "PSA / Warning" Post
Just a heads up for the community. I saw a post asking for a VueScan serial number. I know the software is expensive ($40-$80), but trying to find a free key online is a trap.
How to get your VueScan Serial Number legally
Almost every single "VueScan serial.txt" file or keygen out there contains a Trojan or a Bitcoin miner. I tried three different ones from "popular" torrent sites last week, and my antivirus went crazy.
Just buy the legit license. It follows you for life (even to new computers), and the developer (Ed Hamrick) actually responds to support emails personally.