Take in a full load of Vanguard bounties into the Vanguard Bounty Tracker. I've heard from somewhere (and reserve the right to be wrong) that you have a baseline 2.5% chance of an exotic per bounty handed in, and they stack. If the information is correct, you can give yourself a 30% percent chance of an exotic bounty - if the information is correct. Even if it is correct, it still isn't a guarantee. However, I've had three exotic bounties going at the same time and so having one clearly doesn't stop others from landing.