Hi,
You would install the new receiver/drier before you vacuum down the system. But, you first need to learn how to install A/C components and deal with the O ring seals. Sales people at places like Autozone are often not the best places to get advice. You don't know what type of oil or how much is currently in the system. PAG oil does not mix with R-12 mineral oil if that's what's in there now. It will form a kind of sludge. Also, adding more oil to what may already be in there will lead to too much oil in the system which causes other problems. So, it would be good to slow down, step back a bit and read the shop manual on oil quantity and how to measure it in an RV-2 compressor. PAG oil is also available in different viscosities and off hand I don't know what you would use in an RV-2. I use a polyester based oil for R-134A or 525 mineral oil for R-12
For now don't get sidetracked by discussions on how to charge the system, that comes later. The safest way for someone new to A/C work is to charge the system by gas through the low side service port. You will not run the risk of damaging anything this way.
