Hi Stephen,
Were you able to get LAG working?
I am using the SG200-26 switches from Cisco and I had the exact same problem when the second vmnic was being added by the installer. It would lose connectivity. I didn't see anything suspicious in the switch logs. I had to manually log on to my iLo and disable the standby adapter through the ESXi console (while the installer was configuring networking), in order to make it work.
Now I have just manually configured networking: one vmnic on each port group (VSA-Front End, VSA-Back End, vMotion, and VM network)
I only have 4 NIC's on each host but it's working OK so far. No redundancy at the moment other than the network RAID1.
Did you try posting on the Cisco forum