The following functionality is not available in the current build provided with Vasari. It is available from the source.
I did some work this evening on providing a 3D watch node. The idea behind the 3D watch node is two-fold. First, it should act like the standard watch node, reporting on the results of some branch of the workflow. But, unlike the standard watch node, it draws 3D geometry. It's also partly a kludge to get around having to deal with Revit transactions which are frightfully slow when all you want to do is draw some points and lines. The 3D watch node has, in a perfect world, no transaction overhead because its just drawing points and curves from XYZs and Curves both of which are geometry and not elements.
In the attached image you see my earlier example of a tessellation. One watch is looking at a tessellation on a random distribution of UVs whilst the other is looking at a "square" distribution. And yes, both of them update in real-time (less what it takes to calculate the tessellation) when the slider is updated.