Use the Advanced setting in bananovault and enter the correct indices.
EASY SEED: Add new accounts until you have 20 accounts (0 through 19). You can only have 20 active accounts in BananoVault. Delete an account add the account number 21.
MEDIUM SEED: Reading the documentation on seed index, you notice the recommended index type is an integer. The maximum index is thus the maximum integer on 32 bits, which is 2^32-1 = 4294967295
HARD SEED: Fiddling with the advanced setting, you realize that you can put any number as index and try to generate the account for it. Trying 2^32 works, adding a zero still works, adding a too many zeros generates account with an index less than 10. Some kind of overflow is going on. Searching this way, you can pinpoint the exact index such that the next account index produces an overflow. It is: 999999999999999900000.