While not a common everyday format like .docx or .jpg , the .vac extension is frequently associated with:
If the file is located in a /temp/ or /cache/ directory and the timestamp (May 2021) is years in the past, it is almost certainly a stale artifact. However, if this file is in a production environment associated with security software (like Kaspersky), moving it to a "quarantine" folder for 48 hours before deletion is the safest path. Final Thoughts 1621058650.vac
Associated with Varnish Configuration (VCL) or shared memory logs. While not a common everyday format like