Sous vide is fundamentally about sealing food and cooking it in water. I wouldn't say the precision temperature control is the central part of the concept.
It is possible to sous vide without a vacuum bag in a steam oven, or in certain ovens with temperature probes.
I've cooked 'sous vide' under a running hot tap in a backpacker, which worked great. The water temp happened to be close to perfect (I have a method of measuring temperatures > 40c based on how long I can hold my hand under the running water before pulling out from the pain - surprisingly accurate).
Its about temperature control and efficient thermal transfer; vacuum sealing and using a circulating, temperature controlled water bath are mechanisms of achieving that.