When you land on img.vietqr.io, you are greeted with a single line of text: "From Saigon with <3." That is the entire homepage. There is no explanation of what the site does, who built it, or how to reach the people behind it. For a service that appears to generate QR-code-related images, that lack of transparency is a serious problem.
Most legitimate image-generation or QR-code tools have clear terms of use and a privacy policy. This site has neither. It does not appear on the Wayback Machine at all, which means it is very new or has been kept entirely off the public record. That alone does not prove a scam, but it means there is no track record to judge it by.
The technical side is not bad — fast loading, modern encryption, and clean security headers. But that is like a car that runs well but has no owner listed and no registration papers. You would not buy it, and you should not upload sensitive content to this site until it shows basic signs of being a real business. For now, img.vietqr.io is missing too many of the fundamentals that make a service trustworthy. I would avoid using it for anything important.