To reproduce:
1. Install a full version of SourceGear Vault Professional;
2. Open Unity;
3. Go to Edit > Preferences... > External Tools > Keys > Revision Control Diff/Merge;
- notice SourceGear DiffMerge is not listed there.
4. Install SourceGear DiffMerge individually;
5. Restart Unity;
6. Repeat step 3.

Expected result: SourceGear DiffMerge appears in the Revision Control Diff/Merge list.
Actual result: SourceGear DiffMerge is not recognized when installed via full version of SourceGear Vault.

Note:
-When installing a full version of SourceGear Vault, exe is in:
C:\Program Files (x86)\SourceGear\VaultPro Client\sgdm.exe
-When installing SourceGear DiffMerge individually, exe is in:
C:\Program Files\SourceGear\Common\DiffMerge\sgdm.exe

Fixed in: 2018.3.0a3