For those who want to bet small amounts

Don't immediately claim the money from challenge or daily mission reward chests. Instead, wait until your funds are a bit low and then top them up. If you claim it all at once, your bet size will become too large, so if you want to play within a reasonable range, I recommend this method. (The vault can only hold about 10% of your current balance, so you can't deposit as much as you'd like.)