we need one more life card.
Fill /all/ extra cards with pendulums.
Let's play make believe, shall we?
Every round we have 8 fights, most decks using 6-12 pendulums, assuming the average (9), that's 72 pendulums used per round.
And, seeing as we don't want to discard unnecessary creatures, spells, and permanents, we'll probably wind up discarding mostly pendulums. If we're playing average, we lose 36 pendulums each round, winning half the games and losing half the games. Now, seeing as our vault has 421 cards, we can lose in, at minimum, 4 rounds, and after 3 we'll be squeezed for cards. So assuming we discard only pendulums, we'll need 216 pendulums.
"That's a lot!"
Is it, really? Think back to all the decks you make. Aren't pillars/pendulums usually a third of your deck, or half of your deck, or more? Doesn't that mean the vault should have the same percentage of pendulums? If you were making a 421 card deck, how many pillars would you put in it? In a 42 card deck, I'd put in 13 at minimum, so in a 421 card deck, you need 130, with breathing room. I don't know how many you guys want, but 108? Simply not enough.
(If we go from 108 pendulums to 130 pendulums, it'd take 22 cards, leaving us with 75 cards left to add, 18 with Hrmmm's suggestions. 148 will cut it close, but it should last the war.)