Fairly easy with an UPDATE query with an INNER JOIN.
PHP Code:
UPDATE vb_user AS a
INNER JOIN vb_userfield AS b on a.userid = b.userid
SET a.usergroupid = 8, a.homepage = '', b.field1 = '', b.field2 = '', b.field3 = '', b.field4 = '', b.field5 = '', b.field6 = '', b.field7 = '', b.field8 = '', b.field9 = '', b.field10 = '', b.field11 = '', b.field12 = ''
WHERE b.field2 = 'Afghanistan'