I don't think vdelivermail or vpopmail in general should be calling spamc/spamassassin. Let that be handled elsewhere. Let's stick to delivering mail and deciding where it goes.
However, lets remember that if spam is only scanned at the MTA level, SpamAssassin user preferences will not function if the e-mail is addressed to more than one sender. Scanning in vdelivermail, at the MDA level, does not have this restriction. For that reason I still think there is value in scanning in vdelivermail.