What is the spannig-tree flavor running on your network? PVST+etc.
It could that the TCPdump built-in MARS does not have a parser for that flavor (highly unlikely tough). The 802.1d standard was revised since its original form. Or the MARS NIC card does not suport VLAN tagging, or the port is untagged and no VLAN information is passed to tcpdump (should be like this).
Regards
Farrukh