Transparent mode

Hi Alan,

Transparent mode (Layer 2 deployment) is for a deployment where you need to integrate the FortiGate into an existing network without changing the IP structure, while still allowing for security policies.
Transparent mode does not perform NAT (Layer 3) to the traffics that go through the FortiGate, it simply just do scanning for it.
You cannot assign IP for interface in transparent mode.

For virtual wire pair, can think of it as a pair of interface operate in transparent mode. While FortiGate still operate in default NAT mode.
You cannot assign IP for virtual wire pair interfaces as well.

In conclusion, both modes allow for traffic forwarding without IP addressing.

Some good reading about transparent mode and virtual wire pair:

https://docs.fortinet.com/document/fortigate/7.4.0/ips-architecture-guide/748610/transparent-mode
https://fortinetweb.s3.amazonaws.com/docs.fortinet.com/v2/attachments/5aa37c8a-1a11-11e9-9685-f8bc1258b856/fortigate-transparent-mode-56.pdf
https://docs.fortinet.com/document/fortigate/7.4.4/administration-guide/166804/virtual-wire-pair