So you're in a vacuum with an aqualung, that's the gist of it since you've specified that this tent is airtight, unbreakable and won't collapse under the pressure differential.
The quickest way to die in a vacuum is to have your lungs full of air. The lack of pressure causes gases to expand a lot, so if your lungs are full of air (or any other gas) it will very rapidly expand and cause fatal lung damage. If you breathe out before entering a vacuum you won't die that way because the remaining air in your lungs won't be enough to expand them further than they can safely expand.
If the aqualung remained sealed perfectly to your face, completely airtight, that would help. I don't think it would help enough, though. To keep you live until the air runs out, the kit you're wearing will have to seal you effectively enough to maintain your internal pressure at normal and for that to happen you'd have to be wearing a spacesuit.
Interesting question.