First of all you will never be required to use an Evolution wallet. I'm sure there will be best practices similar to using any phone wallet etcetera and that you shouldn't fill it up with thousands of dollars worth of Dash. The Vault accounts are supposed to be places you can store your money long term. You can do it daily monthly yearly whatever you like but during the time it's in the vault it cannot be accessed. In this way, if you lose your phone or somebody gets access to your account, only the loose funds, those that are available to spend will be at risk. So if you lose your phone or an object that people can force their way into using your pin, you hopefully will have time to secure those funds, but if you're managing the account well, you won't have your life's savings "ready to spend" but in a more secure setup.
Regarding the username and password, I honestly have not looked into it as deeply as I should. You're supposed to be able to retrieve your password. So I'm very curious how they solve this issue. I'm assuming it's all going to be automated and I assume they must also have several types of verification that you have to provide. I don't believe any of this will be managed by human beings. Because of this people will have to be responsible for this information to at least a certain degree. I really need to look into this more. But if this makes you nervous the QT wallet will always be available.