On MW 1.24 and the extension snapshot for 1.24. Trying to Merge and delete users if Delete old user is ticked it gives: "Fatal error: Function name must be a string in ...path/extensions/UserMerge/MergeUser.php on line 255" Without the tick it claims to merge the users, but the old user account is still there.
It's not related to CollapsibleVector. The wiki also uses extension ConfirmAccount. When the user account is created so is a user page. Using user merge with delete selected 1.9.0 gives "Fatal error: Function name must be a string in ..../w/extensions/UserMerge/MergeUser.php". But if the user page is first deleted merge and delete then functions okay.