Hello Anwesha,
You can map Vendor-BP integration through Master Data Synchronisation found in SPRO>Cross-Application Components. You must have created Vendor Account Groups both in FI and in BP. Once BP Role (MKK) settings are done, you can then head over to following for further settings:
SPRO>Cross-Application Components>Master Data Synchronization>Customer/Vendor Integration>Business Partner Settings>Settings for Vendor Integration
You can also make the settings in the way, that vendor be created automatically every time, when a BP is created, provided you have maintained the required configurations steps. However, in your case as you suggested you want to map it on later stage, you can also achieve this using front-end.
Regards,
Amar Lal