I can't think of a reason why you would. I definitely wouldn't.What you need to consider if you replace an all-in-one consumer router that includes WiFi with a modem and OPNsense is that you might need a WiFi access point. Frequently one can configure the former router as such and then place it in the LAN behind OPNsense.
Hopefully most is now clear(er). To add to what Patrick wrote, you wouldn't add a router to the mix if you have one already. A consumer router normally has two or three parts: 1) router ; b) firewall ; 3) optionally a modem.When you have OPN you have 1) and 2). The conversation has been whether you can replace 3).When you put an ISP router in bridge mode, you are disabling 1) and 2) and only using 3)Looking at your purchased Vigor 167, that'll do. Is a modem/router that can be put in bridge mode. The question was if your current one could also be put in bridge mode and save you buying the Vigor.Anyway, seems you're on your way