If it held vacuum, even 20 inHg, for 10 minutes after you stopped running the pump, that is a good sign that you don't have a leak. With that amount of vacuum though I would have expected that it would take the full can of R-134a rather easily. And as you pointed out, if your system is supposed to have 22 oz of R-134a, you wouldn't have a very effective system with only 12 oz in it. Probably the only way you'll know if the charge is right if you don't have a manifold gauge set would be to start over from scratch and put in almost two cans. However, as far as I'm aware, it is still illegal to vent R-134a in to the atmosphere. There is some legal loophole that allows an unlicensed consumer to purchase R-134a, but that still doesn't make it legal to intentionally vent it.
Depending on how badly you want your A/C working, you might be better off to take it to a shop at this point, explain what you've replaced and that you've confirmed the system holds a vacuum. The shop will still need to recover the R-134a that's in it and evacuate the system in order to charge it, but at least they know it will be a tough sell to try to convince you there is a leak somewhere.
-Rod