Hi,

You can inherit account.invoice object and add a functional field.

Try like this:

in .py file:

from openerp.osv import fields, osv

class account_invoice(osv.osv):

    _inherit = "account.invoice"

    def _get_vat_num(self, cr, uid, ids,field_name, args, context=None):

        partner_pool = self.pool.get('res.partner')

        invoice_pool = self.pool.get('account.invoice')

        res = {}

        for inv_id in self.browse(cr, uid, ids, context=context):

            invoice = invoice_pool.browse(cr,uid, inv_id.id, context=None)

            partner = partner_pool.browse(cr, uid, invoice.partner_id.id, context=None)

            res[inv_id.id] = partner.vat

        return res 

    _columns = {

                'inv_vat': fields.function(_get_vat_num, string="VAT", type='char', method=True)  

                 }

in .xml file:

<openerp>

    <data>

        <record id="invoice_form_inherit" model="ir.ui.view">

    		<field name="name">invoice.form.inherit</field>

    		<field name="model">account.invoice</field>

    		<field name="inherit_id" ref="account.invoice_form"/>

    		<field name="arch" type="xml">

    		    <field name="partner_id" position="after">

                                    <field name="inv_vat"/>

    		    </field>

    		</field>

		</record>

    </data>

</openerp>