Validation rule that requires fields to be filled out before change of Opp stage except for particular country

Question

This is the Validation Rule I have created so far, what I need is to include - VAT field on the Account can't be blank except when the Account Country field says USA

AND(
    NOT(ISBLANK( Account.Legal_Name__c)),
    NOT(ISBLANK(Account.Shareholder__c)),
    NOT(ISBLANK(Account.Executive__c)),
    NOT(ISBLANK(Account.BillingAddress)),

    ISPICKVAL(Type,"Partner"),

    OR(
        ISPICKVAL(StageName,"P4"),
        ISPICKVAL(StageName,"P5"),
        ISPICKVAL(StageName,"P6"),
        ISPICKVAL(StageName,"P7")
   )
)