I would always choose two matches with 30% odds vs. one match with 50% odds.
Well, that's a flawed plan.
A vault with a 30% win rate each match has a 2.7% chance to win 3 matches, and a 0.9% chance to win 4.
A vault with a 50% win rate each match has a 12.5% chance to win 3 matches.
Take a guess; which is better?
Light would only want to field two decks if a suicide would hardly help the other deck.
Actually your numbers are giving a certain permutation where the team wins 3 consecutive matches, ending 3-0. There are other possibilities for that team to win, where the match ends 3-1 and 3-2 and in different orders too.
Ending 3-0:
3 / 10 * 3 / 10 * 3 / 10
Ending 3-1 sample:
3 / 10 * 3 / 10 * 7 / 10 * 3 / 10
Ending 3-2 sample:
3 / 10 * 3 / 10 * 7 / 10 * 7 / 10 * 3 / 10
Then you add these values while permutating the 3-1 and 3-2 too (like losing first match then winning 3). Same goes for 50%. The numbers get bigger than you give.
ps. Sorry, at work and i felt like trolling a bit.
ps2: Also you do realize just because you have 2.7% chance to win 3-0 doesn't mean the opponents have 97.3% chance to win 3-0 :S Its actually 34.3%