I forgot to add that this NetVanta unit does not have an NIM; I had read <somewhere> (and cannot find again) I could connect the cable model to an available port and the 1335 would recognize it.
This should work fine. You will actually need three VLANS. The Office and Public LANs and a third VLAN for the cable modem WAN. Configure a port as an access mode for the cable modem on a VLAN by itself, this will be your "Public" security zone. I'd name your Office VLAN zone "Private" to keep with the typical convention, and name the public access VLAN zone "Guest".