Vázquez's two helps of grace through Christ: supernatural and natural
What that necessary grace is, he explains more fully in chapter 12 of the same disputation. There he distinguishes two helps of grace through Christ: one supernatural by its own nature, the other natural. He does not think the first is needed for any work of moral virtue; only the second is, which belongs to the natural order.
His view is that man can do particular works of moral virtue without the help of grace through Christ that is supernatural by nature, the kind needed for the works of the infused virtues. But he cannot do any moral work that is virtuous and good in every respect without some help of grace through Christ that is natural by nature and is prepared for us through natural causes by God's eternal mercy.
Latin
XXIII. Quae sit autem illa gratia quam ad hoc necessario requiri existimat, fusius explicat ipse ejusdem disputationis capite duodecimo. Ubi duplex auxilium gratiae per Christum distinguit, unum quod suapte natura sit ordinis supernaturalis, alterum quod sit ordinis naturalis. Et prius quidem illud necessarium non putat ad quodlibet opus virtutis moralis, sed solum posterius, quod est ordinis naturalis. Ejus enim sententia est, posse quidem hominem singula opera moralis virtutis efficere sine auxilio gratiae per Christum, quod suapte natura sit ordinis supernaturalis, quale ad opera virtutum infusarum necessarium est; non tamen posse ullum opus morale secundum virtutem ex omni parte bonum operari, sine aliquo auxilio gratiae per Christum, quod suapte natura ordinis naturalis sit, & per causas naturales aeterna Dei misericordia nobis praeparetur.