How could I achieve an effect like a volumetric tube extruded along a spline curve and a voxel density decreasing along this spline curve or along V if the rube has a UV system with V following the extrusion curve ?
Somehow I have no Idea how to start on that ..

I did something like that earlier [forums.odforce.net], and I remember having to do it by scattering a lot of points and then doing a Volume From Attribute on those. If there is a smarter way, I'm all ears

E.g. with VDBs:
Map density to curve's points, sweep circle, skin, vdb from polygons by sampling attribute.
Look at file.

Cheers

Wow. I really like the vdb approach .. just tried the vdb analyze node for the first time and its fast !!
Btw. is it possible to access vdb vector fields from dops or do I have to convert them to classic volumes ?

You can use “volumerasterizepoints” sop directly which reads density from source.