Take a look at this blog by Stefan Huebner. He took some measurements with an old P1501 and a couple of Vactrol parts and made a nice chart showing response curves. His conclusion: The VTL5C9 is a much closer match to the old P1501 than any of the parts we have been trying.
http://huebnerie.de/blog/?p=318
Don Backshall
http://huebnerie.de/blog/?p=318
Don Backshall