I can not understand why the
customer_taxvat field in
sales_order table is empty for customers who have Tax/VAT number.
Maybe this can be explained by the fact that there are two different places where customer can add an VAT number. Customer address has a VAT ID field that is stored to the vat_id field in customer_address_entity table.
Customer entity on the other hand has a different Tax/Vat field that is used for example when customer is registering and he is put to certain customer group. This is stored in the taxvat field in customer_entity table.
It is a bit confusing for customers that is for sure.