I’m with @lassoan on this - the “.vtk” extension is widely used in some communities to mean polydata, so people are surprised to see image data in it.
The compound extension idea is working well I think. There are many good examples like .tar.gz', .nii.gz, and in Slicer we have made good use of .seg.nrrd and we’ll probably use that pattern extensively going forward.
So here I’d vote for something like .pd.vtx.