For building the vault, we will likely want to include a few core decks, some splash cards like wings/reverse times and in-element spares. If transmutation rules stay as is, I suggest keeping some room for transmuting towards Seraphs/Nymphs (so having less than 12 of them).
The current war rules has a huge amount of vault discards available (as opposed to deck discards). This means we should pick ALL the 10 cost cards to the max, and discard them as needed:
(9 x 9 + 24 x 1) x 10 = 1050
So for only 1050 points (7-10 nice cards) we can "ignore" 105 vault card discards, pretty cool, huh?
Assuming an average card cost of 100 for the rest of the vault this means ~28% of our vault consists of dead cards. Now, since the discards from vault start at 100% of total discards, dropping to 40%, we should be OK and not ever having to use them, but merely having them for the purpose of discarding.
Depending on the rest of vault, we should consider adding 24 Ash Eaters for the same reason. Upgraded, they can even fill in for pillars to soak up deck discards. Rules changed now, though Ash Eaters could still be nice
Next, because they are OP and used in many fire decks, I suggest we also take max:
Phoenix, Deflagration, Fahrenheit and Fire Bolt.
24 x 100 (= 2400) + 24 x 130 (= 3120) + 24 x 100 (= 3000) + 24 x 110 (= 2640)
2400 + 3120 + 3000 + 2640 = 10960
For off-element cards, same can be said about:
Fractal, Lightning, Dimensional Shield, Shard of Freedom, Wings, Bone Wall and Sanctuary.
9 x 150 (= 1350) + 9 x 120 (= 1080) + 9 x 150 (= 1350) + 9 x 110 (= 990) + 9 x 110 (= 990) + 9 x 130 (= 1170) + 9 x 110 (= 990) = 6980
The card I'm most worried about as Fire is Bone Wall. Now, Fire Bolt can get past Bone Wall, and Phoenixes are OK against it, but it does not hurt having more Bone Wall bypass, like Shard of Freedom (Seraphs, Phoenixes and Crimson Dragons are all Airborne). Still, having more Bone Wall bypassing staple decks apart from the ones I listed in staple deck ideas could be great. Of course, we could also simply field the same awesome decks more than once.