The required parameter works with the fields when you add it in python field definition or XML code. But, it doesn't work when we use a radio widget for the selection field. If you need to make it required, then you have to provide a default value in the definition. Once you give a default value, it will be there and the only option for the user will be to change it and there will be no option to remove it like the normal dropdown option. Hence the field can be used as a required field.
vendor_from = fields.Selection([('user', 'User'), ('vendor', 'Vendor')],
string="Vendor From", default='user')