VLAN or VDOM?

We have a client who runs two businesses from the same office but on two different LANs. They keep the companies completely separate from a technical standpoint but will be sharing a single 100A. They now need a SQL Server for a new application and I' m considering purchasing a single Server and then making that Server available either via a VLAN or VDOM to both LANs. They will go with whatever I recommend. Three options as I see it: 1) VDOM would be nice for managing the separate networks, but I think it would cause more overhead on the Fortinet to throw those packets back and forth. 2) VLAN should be faster and less impact on the Fortinet as it' s somewhat offloaded to the switch? 3) Put two IP' s on the SQL Server and have it live on both networks. Just curious what others would recommend / consider. Thanks in advance.