It seems that nbu to vcf conversion is mainly searched by users that would like to recover their contacts from older Nokia Backups.
You will have to use some of the available utilities for Nokia phones (Noki, NbuExplorer etc.) in order to extract the contacts out of NBU backups and save them in vCard format (*.vcf). There may also be a way to do this using Nokia PC Suite, but it may not work for every version of .nbu files.
Updated: March 9, 2017