FOAF is designed from scratch to be 'Semantic Web'–friendly, ie. to work well with other RDF vocabularies. vCard comes from a different tradition, and was designed with different priorities. There is also a difference in style; FOAF has more 'interesting add-ons'. In brief – FOAF describes people, organizations and their relations, but vCard business cards of these people and organizations.
Proposal: connect FOAF and vCard object using a property term businessCard.