In 64 bit Sonar, why do some instances of Dimension Pro get inserted as "Dimension Pro" and others as "Dimension Pro x64"?
 
There is only one instance of Dimension Pro in the VST scan list.

Do you have an old DX version floating around? I do not. I have tried adding DP from the main menu, browser and the add track menu. They all insert the synth as "Dimension Pro" with an instance number appended.
 
The path to the dll is in the VST details displayed using the Details button in the Properties dialog called from the VST drop down in the VST standard header.

The only DXi is Dimension LE and there's only 2 Dimension Pro.exe files:
 
C:\Program Files (x86)\Cakewalk\Vstplugins\Dimension Pro\Dimension Pro 1.5.exe.dll
C:\Program Files\Cakewalk\Vstplugins\Dimension Pro\Dimension Pro x64 1.5.exe.dll
 
So the ones not marked "x64" must be 32 bit saved from an earlier 32 bit Sonar instance.
 
The exe file named "x64" must have been renamed such by me.

SONAR does not use the exe files as plug-ins. Those are the stand alone minihosts for the synth. SONAR only loads the DPro plug-in files. I do not recall if the 64bit VST dll was always called "Dimension Pro x64.dll" but that is its current name. Also messing with VSTPlugInNameFormat in Cakewalk.ini affects plug-in names in SONAR.

Right. I meant .dll not .exe. Both are in each folder. If I rename plugs I do it in Plugin Manager.

In Sonar Platinum (64-bit), if I look in the Plug-in Manager, I only have one entry for Dimension Pro with the file path:  c:\program files\cakewalk\vstplugins\Dimension Pro\Dimension Pro x64.dll
 
On the other hand if I open Sonar Platinum (32-bit), I will find Dimension Pro listed with this file path:  c:\program files (x86)\cakewalk\vstplugins\Dimension Pro\Dimension Pro.dll
 
These are the stock installed names and paths for my setup.  I have limited the VST scan paths for my Sonar (64-bit) so that it picks up only a very select few 32-bit plugins, which excludes the 32-bit Dimension Pro.

So I must have started the project in 32 bit and then added to it in 64 bit to get the two different Dimension listings.