Discovered via Support ticket 4357 from BlueCat Networks.
https://support.isc.org/Ticket/Display.html?id=4357

Upon examination:  dhcpd does not catalog virtual ethernet interfaces during
interface discovery and thus will not listen or offer service in a subnet served
only by a virtual ethernet interface.

May also affect Solaris:  see Bugs ticket 17965:
https://bugs.isc.org/Ticket/Display.html?id=17965