Smoke ring in space? I understand that a smoke ring is an example of a vortex.  I sort of grasp why a vortex stays coherent - the rotary motion of component molecules cause a pressure difference within the vortex.
I do not understand whether there is some interaction with the external environment that also keeps the vortex stable.  
The question: would a smoke ring blown in a vacuum stay a coherent ring?  Or would the ring tear itself apart immediately without opposing external molecules to hold it in?   
A related but less extreme scenario would be a water ring "blown" into zero-gravity air.  
 A: The smoke ring is an instance of vortex, as you have mentioned. So, to begin with - 
How does the vortex work?
The vortex effect is possible due to the viscous friction between the substance you inject and the environment it's injected in.
How does the friction affect the injected substance? Its outer layers slow down:

But the inner ones are still moving fast compared to the outer ones. A moving object will create a temporary deficit of pressure behind it; and the outer layers, which have been overtaken by the inner layers, are no fools! They immediately follow in the territory of low pressure. This way, the outer layers are "spinning up". 
 I don't think it's necessary to analyse this in a more detailed way for this question 

I hope that this brief introduction is enough for your question to be answered:
Is it possible for the smoke ring to form in space?
The answer is: nope.
In order for such a ring to exist, there should be an environment which will cause the friction and slow down the outer layers of the injected substance.
As the friction in the space will be almost inexistent, the outer layers will not slow down and spin up (to a degree when the vortex effect could be noticed by you).
