If the mtu is 4 byte smaller it most likely has to do with the vlan128 you have on the wan.
This will be the vlan tag: Two bytes are used for the tag protocol identifier (TPID), the other two bytes for tag control information (TCI)
So this indicates you will have a subinterface 128 on your wan, and the default gateway is .178.
I would expect this at the provider side, but you wrote you hit the webinterface of the vigor at that address.
The subnet you have with 255.255.255.248 mask has network address 178.x.y.176 and usable adresses are .177 to .182
Normally a default gateway woud be first usable address. (.177)
Can you find a static route to 0.0.0.0/0 in the vigor pointing to 178.x.y.177 ?
The ip addres the vigor gets via dhcp might only be for fiber to manage the device through..
What you could try with pfsense is to make the wan ip address .178 with subnet mask 255.255.255.248 (in the subinterface 128)
You than would need to make aliasses for the public adresses on the wan side, and nat your internal ranges to these aliasses.
So
192.168.1.0/24 to .181
192.168.2.0/24 to .180
192.168.3.0/24 to .179
See this: https://forum.pfsense.org/index.php/topic,64387.0/topicseen.html
I think you not need that dhcp stuff at all, as the vigor is not longer used.