My guess is that the Safe/Vault are much like my insulated keg. IF that's the case the 2 things you should keep in mind to prevent temps from not running away are:
Start with a small fire (in my keg it's an area slightly smaller than a baseball). It's always easier to stoke a fire then it is to get it smaller. Also don't put any wood chunks directly on the lit area.
2nd thing is you have to nurse the temp. up slowly. It takes time to heat up all that metal and the heat slowly accumulates in the cooking chamber, remember the insulation is doing it's job. It takes longer this way to get up to temp. but that's just part of the cooking process for me anyway. I don't use water in my keg, but IMO water seems to complicate the matter. I'm sure others have figured this out though.