A explanation/work-around for this issue raised back in April.
It seems that if, in UCS, you configure a vNIC with a single native VLAN it will still add an 802.1q header with tag 0 - possibly to do with QoS. And this extra header prevents iPXE from parsing the DHCP response.
The solution for me was to present all VLANs on a single trunked vNIC to the blade and configure VLAN tagging as per normal. The result is the tags are stripped off the packets before being passed to the VM and DHCP now works.
The same issue applies to VM-FEX as packets coming off the VF will have the 802.1q header. The only solution I can see here is to configure a bridged interface for initial build of the VM and then switch to VM-FEX afterwards.