I'm not sure I fully understand your current topology.
If you are not doing iSCSI boot from san, I would think you could simply modify your current vnic that carries iscsi traffic to allow/trunk your additional/new iSCSI vlan. Then add an additional vmk iSCSI port bound with new vlan to the same vmnic.
Ideally, you would create an additional vnic, with just the new iSCSI vlan, and have the additional VMK iSCSI port bound to it's corresponding vmnic.
Kirk...