I know that most of the procedures say "never mix oils" but from actual field experience this is rarely a problem, in that PAG and the more traditional R12 lubricants do mix together well enough-- too many DIY conversions from R12 to R134a without any issues to believe the hype that you must flush out the old R12 with solvent before introducing R134a and PAG.
That being said?
A vacuum is really required, as you cannot permit any moisture in the system-- it'll (moisture) ruin the compressor, and drawing a vacuum for at least 30 minutes is not only a good idea, but is a must-- go longer, if you're working on a high-humid day. There is no such thing as "too much vacuum" but there certainly is too little!
The vacuum's purpose is to boil off any moisture in the system-- water boils at low pressure, remember, and 30 inches of vacuum (the theoretical maximum) will boil water at room temperature.
As for how much oil? That's a variable that depends on a lot of things, but if you cannot locate the exact value anywhere? Plug (with blue tape) the outlet on the compressor, and pour in PAG until it's full, slowly turning the shaft by hand. At the very least, you'll have filled up the compressor, ensuring it has plenty of lubrication.
Some factory or rebuilt compressors come pre-charged with oil-- it never hurts to ask your supplier if this is so-- and he may well have the spec on how much to use, too-- I've seen as low as 2 ounces, which is not much at all.
I also prefer to use the PAG with the lovely UV die already mixed in-- why not? It makes spotting a leak much easier than otherwise.
If you have the spec, and it calls for 7 ounces (say), and you can only get 1 or 2 ounces into the compressor before it overflows? Not to worry-- you can suck it in with your vaccum pump-- connect one of the two hoses to the pump, through your manifold guage set, and leave the other hose not connected to the manifold (be sure to close that valve). Start your vacuum pump, open the valve with the hose, and take the free hose (attached at one end to the vehicle) and dip it into the oil, letting the vaccum suck it up. I like to suck from the high (blue) side, and draw the oil into the low (red) side-- this means the oil will flow directly into the pump. When you've drawn in all the oil, close the valve (let the pump run, it won't hurt) then re-attach the free hose to the other side of the manifold-- wipe it on a rag first! Leave that side closed for now, though. Re-open the other side valve, and let the pump work a bit, drawing the oil through the pump. After few minutes, open both valves fully to the stop-- they tend to leak if you don't open them all the way.
Let it run for at least 30 minutes or even an hour if it's humid.
Obviously, you're going to need to weigh the charge in, once you've completed the vacuum-- there should be a charge sticker underneath the hood--- go by that, in either ounces or grams/kg.
Automotive systems are carefully balanced to work with variable speed compressors (engine driven, remember?) and you really cannot get an accurate charge just going by pressure/temperature differentials. Which is why weighing in the charge is a must.
However, if you find the system does not seem to want to "take" the full R134a charge? (as specified by the label).
Simply revving the engine will increase the suction action on the low side, drawing in more charge-- too much, if you let it.
Okay, I just checked-- my 2004 North American van had the sticker at the right (as you face the engine) up on the rail-- it reads:
1.9 LB R134a
0.890 kg R134a
Lubricant/OEL 001 989 08 03
This last is likely a part number for official MB compressor oil-- a good parts house should be able to cross reference that with a suitable duplicate, and likely also give you the proper amount too.
Good luck!