Hi!
Your best chance is to load the vtu file in paraview and use "save data" to save it in the "Legacy VTK" format. This worked for me, but you milage may vary, as the different Paraview versions save to different versions of the "legacy" VTK format. Meshtool supports several, but not all legacy VTK formats. Unfortunately, the VTK people keep updating their supposedly "legacy" format, which defeats the whole purpose of a legacy format.
Good luck, Aurel