Further to my second thoughts mentioned above, after more deliberation I changed my opinion about the OBi110. It should be capable of doing the job but might require a small workaround.
The OBi110 is perfectly capable of forwarding ("bridging") an incoming call on its LINE port directly to the IP address of the VTech. So receiving calls is OK.
The potential issue is with outgoing calls. The VTech cannot SIP register to the OBi110. Some IP phones cannot make outgoing calls if they are not SIP registered. If the VTech can, then no problem. Configure it to send ougoing calls to the OBi110 and configure the OBi to bridge to the LINE port.
If the VTech cannot make calls if not SIP registered there is a workaround. An ordinary analogue phone plugged in to the PHONE port of the OBi can be used to dial the call, then hook flash and dial a speedcall number configured on the OBi to call the VTech's IP address. Then hang up the phone and the call continues between the VTech and the distant party.
Even if you don't need the analogue phone to make outgoing calls, you may want one anyway with either an OBi110 or OBi212. The OBi can be configured to have incoming LINE calls ring both the analogue phone and the SIP phone. Answer the call on either. And choice of which to use for outging calls.