I am also unable to uninstall the regular Play Vendor with Titanium Backup (crashes), Root Uninstaller (no error, but apparently fails, because the install error remains the same), nor install the mod with Root Explorer (gives "same name with conflicting signature" error).
Would having the Play Vendor app not listed as a system app do the trick?
EDIT: I also tried mounting the system partition R/W and renaming the app, then naming the modded version as the same app, and restarting. This just left the app unavailable. Installing it failed, but without saying way.