Thanks for your information

but still, I'm confused, why This button always visible.

i can say funny, but where are a lot of places where can be 'strage', but when odoo trying to cover all part of life, no part will be covered very good. it is quite big problem, when you try to implement full scope(sales/inventory/purchase/accounting) and you get a lot of time such things, whre users can make mistakes, but odoo allows it.

Thanks, Primus creativus,

For this button, Some time users accidentally create another Vendor bill, Even the first one already done.

I know, but still, where is only way to fix-made customization :)