Using theme hook suggestions for webform confirmation page
I think your problem is due to the use of $vars['theme_hook_suggestion'] (no trailing s) by webform.
This value overrides any that you could put in the $vars['theme_hook_suggestions'] array

I was having the same problem, I just unset this $vars['theme_hook_suggestion'] and my others suggestions in $vars['theme_hook_suggestions'] have been used.

Note: $vars['theme_hook_suggestion'] is a string NOT an array