I have a problem with a permalink og custo post type. Rhere is no way to see that custom post type or edit that custom post type with toolset form. I changed severl times parmalink structure. It works with all custom post types bit not with one of them. If I change the slug of that custom post type... it works fine, but if i regret to the original slug... it doesn't work. I don't now if that is caused because the slug hav only 2 letters... but there is no way that see that custom post type in his content tempate or edit with toolset form.
I deactivated all pulgins... and the problem is there...
The problem seems to be stemming from your server and i'm unsure why that is.
I've taken a copy of your site and deployed it here locally and the URL slug with 'vi' works. Unfortunately i can't do much in the way of server debugging. However is it possible for you to use a slightly modified slug ?