I've had Vault for a little while. They're great smokers!
As for adding fuel (charcoal, wood, etc.), I haven't had to add anything during a normal smoke. Please keep in mind that I cook "dry", meaning I don't use the built-in water pan. I have no problem getting 14+ hours out of a single loading of fuel. If you use a weed burner to pre-heat the Vault/Safe, it will increase your cooking time(s) greatly.
I hope this helps......