Sorry, I think I misunderstood at first reading. You may try to combine vector with raster using Inset:
cnpl = ListContourPlot[RandomReal[{-1, 1}, {30, 30}],
InterpolationOrder -> 2];
plgs = DeleteCases[cnpl, _Line, All];
new = ListContourPlot[RandomReal[{-1, 1}, {30, 30}],
InterpolationOrder -> 2, ContourShading -> False,
Prolog -> Inset[Rasterize[plgs]]]
The code needs some adjustments, but I don't have time now.