screen customization : unable to mask VAT in order to provider

Hello,

i'm trying to mask VAT flieds in order to provider (so, we pay VAT on every expense and don't need to compute it).
It is impossible for Glovbal VAT in order to provider screen (i can set it but it vanish when saving) and even mask it by order type it remains visible on order screen.

Here is the customization screen :
 

Here is the order to provider screen :
 

Regards,