Can adapter be plugged into a different bus?

I have an 04 Freightliner. It has a VDC in back that is a receiver on the same serial bus (J1708) that the Bluefire is plugged into. The VDC adds the air pressure data and potentially other bits. It sends that data via a different serial bus (RS-485) to the Message Center (MC) in the dash. The MC adds in fuel level and outputs yet another serial bus to the gauges.

My goal is to eliminate the MC and the other gauges to be replaced with a bluefire dashboard. Is it possible to plug the adapter into the gauge, or MC databus so that I can have the fuel level and air pressure gauges?

Alternatively, if the VDC is not forwarding all the data from the J1708 or the adapter is not compatible with the other serial buses, then can I put an RS-485-USB adapter into the gauge bus and pick off the data I want to add to the bluefire data so I can use the same gauge UI?

I am a software developer, so I can do whatever programming is necessary.

The adapter can only retrieve J1708 data and the App can only receive data from the adapter, sorry.

Can you elaborate? Is the J1708 different electrically and/or protocol from the VDC/Message bus?

J1708 is a SAE standard messaging format. The hardware protocol is J1587 which is essentially RS485 but with the SAE J1587 specifications. If the other buses adhere to the J1587 and J1708 specifications then the adapter will work on them.

I managed to make sense of the data going from the VDC to the message center. But to further test, I put my RS-485 reader into the 6 pin connector that the bluefire connects to (with 9 pin adapter) and failed to make sense of the data.

My understanding is that the 6 pin connector should have J1708 RS-485 9600 8N1 and J1587 message format as described in https://cdn.hackaday.io/files/18651797964384/SAE%20J1587%20(2002%20Standard).pdf

Is it going at a different baud or bit format than 9600 8N1?
Your adapter will interpret the higher baud of J1939, but that arrives on different pins, right? So you listen on both, and I assume you set the older pins to be 9600 8N1, but maybe you have to try different baud rates?
Or maybe there is some additional message format for MIDs below 128 that I need to interpret?

Nope, you're right on with the baud rate and message format. It is a constant stream of data though.