I am a DIY'er and I replaced my evaporator myself. However, I took the car to a professional with the right equipment for evacuation and convertion to r134. By the way, I don;t think you have r134 in your car. I think you may still have R12. Check the labels in your engine bay.
The vacuum elements control the opening and closing of the flaps to direct air condition flow within the cabin. Once you remove the dash, they will be seen at plain view. Easy to replace. You can buy the whole thing or just the rubber portion, which goes bad after a while. Do a search, it has been discussed recently.
As far as the sealant once you open the "box" you will figure it out. It is just a regular foam seal similar to what you use for a window ac.
I wish you luck!