Two of my machines running Vegas Pro 13-428 on Windows 7 Professional use lowercase .mp4 extensions for the Mainconcept and Sony AVC/MVC render templates.
I even rendered to a USB drive that was formatted FAT.
I looked through Vegas Pro Options including Options / Internal and could find no reference that seemed to affect the case of the file extension.
All of my cameras name every file in all uppercase. Bugs the snot out of me.
Windows' bulk renaming feature will allow me to change the whole lot of them to a lowercase name in one step, but it keeps the extension uppercase, even when i've retyped it as lower. Why???
the extension case upper lower should have nothing to do with Sony Vegas. It's all about your OS and file association. Buried deep in the association that began when the file type became first clocked - i.e., that is now in your registry as that style. If you can't find the interface, then you edit the registry.