i think the vault upgrades should start doubling from the current max at 5 casinos open no matter what the current income and every successive 5 after that should be able to double again. and if the doubling gets rediculously high after awhile i think it should scale per 24 hours based on your income like. if you have TONS of income it should allow it to gain for AT LEAST 25 hours minimum before needing to clear vault.