Good question!

Since I already had NeatVideo for VirtualDub, when I installed VPX5 and pointed it to the folder where VDub plugins are stored, it immediately found and was able to use it.

So, although there is nothing on the website or in the help files that I can see, I think the "route" to acquiring this very useful plugin is: Download and Install the (free) VirtualDub (it has a number of plugins available which can also be seen and used by VPX); download and install the trial of one of the versions of "NeatVideo for VirtualDub" (the Home version is quite adequate, unless you deal with HD Widescreen footage, in which case the Pro version is what you need.)  Test it in VPX.  (It appears under Effects>Extra Effects>plug ins (though you will need to point the plug in manager to the folder where you have installed the plug in(s) - I put NeatVideo and all the other VDub plug ins in a plug ins folder within VDub so it's then available to both VPX and VDub - to select this folder in VPX you will need to press the "Advanced" button on the plug-in manager screen.)  If you like the result of using the plug you will then have to buy it in order to continue using it!

HTH

Jeff

Much later edit: I actually wonder, on reflection, if you need to download and install VirtualDub?  If you simply have the plug in files and point VPX to where they are stored, I think they will be "seen" and "used".