You need 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.
Convert Nokia PC Suite 6.82+ backup to vCard contacts.
We found 2 software records in the database related to .nbu to .vcf file format conversion.
Microsoft Windows software - convert nbu to vcf on Windows
A small usefull program for exploring and extracting of files created with Nokia PC suite
Convert from nbu file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned nbu to vcf. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A free explorer for Nokia NBU files