See https://svnweb.freebsd.org/base/head/sys/dev/vnic/nicvf_main.c?view=markup#l498 and l516. nicvf_set_multicast is not implemented.
Note that the Linux driver which was used for porting (Cavium provided a BSD licensed version) also uses promiscuous mode: https://github.com/torvalds/linux/blob/master/drivers/net/ethernet/cavium/thunder/thunder_bgx.c#L889
A commit references this bug:
Date: Tue Nov 14 16:31:12 UTC 2017
New revision: 325811
vnic: report that the driver supports multicast
The driver is currently hardcoded to force promiscuous mode, so all of
the MAC filtering code is presently unused and multicast should "just
work." Report to the higher layers that multicast is supported.
Reported by: bz
Sponsored by: The FreeBSD Foundation, Packet.net (hardware)