Oh, the system 'works', it's just you get very little from each individual card. Unless they changed the math in the last few months, it was like 4% per 5th mythic, 1% per 5th rare, and something like .1% for uncommons. I just remember you needed 100 'extra' rares to open a vault, which is ridiculously small.
That said, at the end of beta I had most cards and was crushing most drafts (specifically, I was good at m19 and DOM), so I was still opening one vault every 4-6 days that I played. Basically, I wouldn't suggest anyone spend money on packs, just drafts. Even if you can't get 4+ wins consistently, I believe it still works out to a lot more cards per dollar. I don't have the exact math anymore, but I vaguely recall it being cheaper than buying packs even at an avg of 1.5-2 wins. So, unless you hate draft with a passion, it's the best way to build a collection on mtga.
edit: OH, also, once you have the specific commons/uncommons you need for decks, spend the wildcards you get on whatever you don't have 4 copies of yet. That way, when you open them in packs you get a little more vault progress. You will eventually build up a huge surplus of common/uncommon wildcards, so might as well funnel them towards getting rare/mythic wildcards instead.
