if i invoice a customer... and my invoice has the customers vat number under the customers details...but my invoice is labelled "invoice" and not "Tax" invoice and i dont put a VAT number on my invoice under my details is this legal or must i leave off the customers vat number rather?

when i invoice customers via my other company which is vat registered...i have both my vat number and the customers vat number at the top of the invoice in seperate blocks.