First off r12 is a ozone depleting gas and r134a is a green house gas two
very different areas there are many way's to convert in a vanagon or westy I
would flush the entire system of old r12 oil as the two oils are not
compatible then put the oil charge that the automobile needs remember we are
running 20yr old systems everything needs a oil change now and then. When we
first started doing conversions there was many opinions on how do to do one
properly from changing all the o-rings out and flushing the system  and
installing r134a fittings to just poping in a once of pag oil and doing a
90% fill of r134a to the system both options do work but my preference is to
flush and fill with new proper oil new r134a dryer and a few seals do a
pressure test  to check for leaks and then vacuum and refill