Use one2many or many2many field for that purpose. Both might be computed based on a date. Both might be simply shown on a view just by adding <field name='vat_ids'/>

The example if the VAT model is linked to a partner:

vat_ids = fields.One2many(
    "vat.model", # the vat history model
    "partner_id", # the link to partner on each line, which is updated when you update partner VAT
    string="VAT History",
)

The example if the VAT model is not linked to a partner and should be somehow computed:

@api.multi
def _compute_vat_ids(self):
    for partner in self:
        vat_ids = [1, 2, 8] # use some logic to find vat history items by dates and partner
        partner.vat_ids = [(6, 0, vat_ids)]

vat_ids = fields.Many2many(
    "vat.model", # the vat history model
    compute=_compute_vat_ids,
    string="VAT History",
)

UPDATE

You have stated the comodel not correctly. You should apply Odoo name, not Python name

class Vat(models.Model): 
    _name ='utilitypower.iva' 
    _description = 'Iva' 
    _rec_name = 'descrizione' 

    descrizione = fields.Char('Descrizione') 
    valore = fields.Integer('Valore',required=True) 
    anagrafica_vat_ids = fields.One2many(
        'utilitypower.anagraficaiva', # NOT utilitypower.ClientsVat
        'id_vat',
        string='Anagrafica Iva'
    ) 

class ClientsVat(models.Model): 
    _name ='utilitypower.anagraficaiva' 
    _description = 'Anagrafica Iva' 
    
    id_vat = fields.Many2one(
        'utilitypower.iva', # NOT utilitypower.vat
        string="Iva",
    ) 
    id_client= fields.Many2one(
        'res.partner', # it is fine
         string="Cliente",
    )

#PARTNER

    anagrafiche_vat_ids = fields.One2many(
        'utilitypower.anagraficaiva', # NOT utilitypower.ClientsVat
        'id_client',
        string='Anagrafica Iva',
    )

P.S. The next time please use 'code' tag to put code. Otherwise, it is not readable at all. 

Also added a like in order you can comment.