Please consider adding: @NpmPackage(value = "@zxing/library", version = "^0.18.4") to the ZXingVaadinReader classes. Then Vaadin will automatically add ZXing to the package.json and will install it when the application starts. Then no manual npm install ... is required.
This should also fix the heroku problem. Your other projects are in public github repositories. Having a public repo for bug reports and pull requests would make it easier.