There is only one file type that uses the .VSU extension:
Visio Custom User Interface File
A VSU file contains information about a custom user interface (UI) for Microsoft Visio, a program used to make drawings and technical illustrations. It stores information about various UI elements, such as menus, toolbars and status bars. VSU files are often used to save custom user interfaces created by users.
The VSU file type was introduced with the release of Microsoft Visio 2000. When a user modifies a custom UI, the VSU file is not updated until the program is closed.
In Windows XP, VSU files are saved in the following directory:
C:\Documents and Settings\[username]\Application Data\Microsoft\Visio
In Windows Vista, VSU files are saved in the following directory:
Updated: December 2, 2019