We have found 2 software records in our database eligible for .nbu to .vcf file format conversion.
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.
A small usefull program for exploring and extracting of files created with Nokia PC suite
NBU
Files with nbu extension can be usually found as old Nokia phone backups made using Nokia PC Suite 6.82 and higher.
VCF
Files with vcf extension are almost always contacts saved in vCard format. Typically used various Personal Information Managers or e-mail clients (for example Outlook), as well as exported from mobile phones, for the purposes of sharing or migrating contacts from one device to another.