Users looking for nbu to vcf conversion are typically searching for a way to recover their contacts from older Nokia Backups. For this you have to use one of the Nokia utilities (Noki, NbuExplorer) that allow users to extract and export contacts from NBU backups and save them in CSV or vCard format.
Open NBU file Open VCF file
A small usefull program for exploring and extracting of files created with Nokia PC suite
A free explorer for Nokia NBU files