When is VAT deductible?

My accountant told me that in order to subtract VAT from a purchase, my VAT number needs to be present on the invoice.

Let's say I purchase an item in Belgium that is used for professional means and the VAT is mentioned separately on the invoice. I ask the supplier to make an invoice for my business, but they refuse saying that "they don't sell to professional customers". (happens more often than you think)

A friend of mine suggested to add my VAT number as the first address line of the "invoice address" and the "street and number" as the second address line. This way, it looks as if the VAT number was printed deliberately. Is it a valid invoice? In other words, is the VAT fully deductible? Thanks in advance!