partner-contact/partner_contact_vat/view/res_partner.xml

<?xml version="1.0" encoding="utf-8"?>

<!-- © 2015 Grupo ESOC Ingeniería de servicios, S.L. - Jairo Llopis
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html). -->

<openerp>
<data>

<record id="partner_form" model="ir.ui.view">
    <field name="name">VAT for contacts</field>
    <field name="model">res.partner</field>
    <field name="inherit_id" ref="partner_contact_personal_information_page.personal_information"/>
    <field name="arch" type="xml">
        <data>
            <!-- Accounting page -->
            <xpath expr="//group[@name='personal_information_group']">
                <group>
                    <label for="contact_vat"/>
                    <div name="contact_vat_main">
	                    <field
	                        name="contact_vat"
	                        placeholder="e.g. BE0477472701"
	                        class="oe_inline"/>
	                    <button
	                        colspan="2"
	                        name="contact_vat_check"
	                        string="Check Validity"
	                        type="object"
	                        icon="gtk-execute"
	                        class="oe_inline"/>
	                </div>
                </group>
            </xpath>

            <!-- Contacts subform -->
            <xpath expr="//page[@string='Contacts']
                         //form[@string='Contact']
                         //field[@name='mobile']"
                   position="after">
                <label for="contact_vat"/>
                <div name="contact_vat_subform">
                    <field
                        name="contact_vat"
                        placeholder="e.g. BE0477472701"
                        class="oe_inline"/>
                    <button
                        colspan="2"
                        name="contact_vat_check"
                        string="Check Validity"
                        type="object"
                        icon="gtk-execute"
                        class="oe_inline"/>
                </div>
            </xpath>
        </data>
    </field>
</record>

</data>
</openerp>