Scenario One
If the vials aren't shattered exactly simultaneously, then it works as follows.
Step 1: Each Player rolls 1 DC 20 save. If he passes, repeat this step for the second and subsequent saves, returning to this step after each successful save. On the first FAILED save, deal onset damage and proceed to step 2. If all 6 saves are passed, this sequence terminates.
Step 2: Having taken the onset damage, the player turns to the remaining doses and begins making saves against them. The save is now DC 22, as he has one dose of poison in his system. If his saves pass, he continues making DC 22 saves against remaining doses until he has passed all remaining saves, returning to this step after each passed save. On the second (and subsequent) FAILED save, proceed to step 3.
Step 3: Deal onset damage. Then, for every failed save after the first, increase the DC of all remaining saves against the poison by +2 and increase the duration by 50%. Return to step 2 after increasing DC and duration.
Scenario Two
If the vials are shattered exactly simultaneously, then it's as follows:
Step 1: Each Player rolls 1 DC 30 save. If he fails, he takes the initial onset damage and is poisoned for (normal duration +50% x5). The saves during this duration are all DC 30. If he passes, he is not poisoned. End of sequence.
Poison Cloud
For the poison cloud afterwards, it's debatable if it counts as a concentrated dose of the poison. If you think it does, that's a DC 30 save each round to avoid either a) onset damage and being poisoned, or b) onset damage, +2 DC and +50% duration if already poisoned. If you think it doesn't count as a concentrated dose, it's as above, but only DC 20.
Poisoned Characters
Poisoned characters then have to save during each of their turns to avoid ongoing poison damage, at the DC set by how many doses they have in their system, and for a duration based on the number of doses.
Stray Thoughts
The initial save can be describes as 'Does this dose of poison enter my system?' If the answer is YES, then check 'Am I already suffering from one or more doses of this poison?' If the answer is NO, deal onset damage and the character is poisoned. If the answer is YES, deal onset damage, +2 DC to the existing poison, +50% duration to the existing poison.
Paizo Blog on poisons may also help.