Yep, you are right to be fearful of 40 year old plastic parts....which is why I have a 3d Printer !!.
However, The side vents have a single screw at the side of the vent in the door Jamb and then there are 4 sprung plastic tabs on the front plane of the vent that holds it in place.
I used a cheap wide plastic trim removal lever to lift and hold the vent up as much as possible and then used a large flat blade screwdriver the lever the vent forward. I mostly did this with my eyes closed !!
The second vent was easy knowing how to do it.
The replacement control bar for the vane just snaps in and the whole vent simply clicks back in place
I can post the control bar in the 3d print section of the forum if you can find someone with a printer to make it up. Here in Oz there are little boutique 3d print shops in shopping centres who will do small print jobs for you.
Cheers,
Steve