Nokia NbuExplorer not only works with NBU but also NFB backups and can extract the data from it. So you can use it to export the contacts from the backups and save them in vCard (*.vcf format).
Convert Nokia PC Suite phone backup file to vCard file.
We found 2 software records in the database related to .nfb to .vcard file format conversion.
Microsoft Windows software - convert nfb to vcard on Windows
A small usefull program for exploring and extracting of files created with Nokia PC suite
Convert from nfb 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 nfb 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