Vigorish on parlays

Let's take a 3 team parlay for example that pays 6 to 1 (or it may be 6 for 1?). I'm not sure, but its 500 total profit. What's the percentage of vigorish you pay on that versus the same parlay that pays 9 to 1 (900 total profit) with 20% vig on a loss? What's the better bet?

It's probably noteworthy to add that the 2nd option is actually a round robin using 3 parlays for 2 teams. Meaning I can win some money if 2 of the 3 teams win. Typically a round robin in that format pays 2.6/1. This is paying 9 to 1 with 20% on the loss.

A +500 three team parlay has an 8.2% commission on each game, provided each pick has a 50% chance. For parlays where there is better than 50% chance, like a ties win parlay card or a northern Nevada baseball parlay card it will be less