Several utilities for Nokia phones allows users to export contacts from phone backups (*.nbu) and save then in a vCard format (*.vcf). Should be easy enough with programs like Noki or NbuExplorer.
Convert Nokia PC Suite 6.82+ backup to vCard contacts file.
We found 2 software records in the database related to .nbu to .vcard file format conversion.
Microsoft Windows software - convert nbu to vcard 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 vcard. 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