AC leak repair and vacuum time for recharge

You have been watching a lot longer than we do. If vacuum holds for ten or fifteen minutes, we charge the system fully, then check for leaks with an electronic leak detector. You have to consider that your vacuum pump and hoses have been added to the system you are looking for leaks in. Also, the valves in the ports commonly leak. While that is not a concern here, the valves' only purpose is to hold the refrigerant in long enough to give you time to put the caps on.

If you are losing vacuum in just a few minutes, that can be a sign there is still water in the system, and not necessarily a leak. Water boils at seventy seven degrees in a vacuum, and that makes it expand. That will cause the vacuum reading to go down.

The accumulator should be fine unless it was open for more than a couple days, just pull a good vacuum. The vacuum is for a couple reasons, one it will pull the air out of the system, air contains moisture and undesirable gases you do not want in the system as they will create an acidic mix that can corrode the aluminum parts and damage the compressor. Two it will lower the pressure in the system and "boil" the water out of any spots it has accumulated like the desiccant or in the oil that remains in the system.
The trick to the vacuum test is that you need to remember that all of the seals in the system are designed to hold pressure, not a vacuum. Some seals can do both but the compressor seals will almost always leak some, just the nature of the design. Some of the OEM hose coupling designs will leak unless they are perfect, the quick connects used on Jeeps and Fords that use the spring clip that you just push together are a real piece of work.
Another thing to watch for is flaking paint on parts like the evaporator and condenser, if the system leaks through any corrosion under the paint, the paint can act like a check valve and hold under vacuum but not under pressure. I use a wire brush to remove any suspect paint/coating to be sure there isn't something hidden.

I normally pull a vacuum and check it after twenty minutes or so. If it has not dropped I will charge it with dye added, look it over with UV and sniffer and if it is okay ship it. If I find something I will pull the charge back out and repair it.

Air is always a good thing for keeping cool and staying alive. Good luck. Keep us informed with the results.