"Vendor keys :modified" is being shown in uefi secure boot

Secure Boot works by placing the root of trust in firmware, usually via x509 certificates.

A root CA is embedded in firmware such that it can then validate the signed bootloader, the signed bootloader can then validate the signed kernel or signed second stage boot loader, and so on. Various key databases in the firmware are used to provide flexibility and maintain strong security.

The word keys here means certificates. Microsoft has its own key/certificate, and so do computer vendors. The certificates/keys are stored in the firmware and used to verify its contents or any loaded software.

In your case, you have modified some BIOS variables by enabling secure boot. The change was verified and then signed again by the vendor key(s), which will also be required if you decide to disable secure boot. "Modified" means that you have done some modifications which were verified/signed using vendor keys.