This might be because of a conflict with another program that also has installed Qt4 libraries, and that these libraries can be found in your path at a location that takes precedence over VNL.
I know myself of one such program, which in a non-standard way inserts itself in the beginning of the path, unlike the standard behavior to add yourself to the end of the path, namely MIKTeX.
If you have MIKTeX installed, move its bin directory to the end of the path and try VNL again.