Why Vázquez calls this natural help grace of Christ
That is why he contends that this help, needed for any good moral work, belongs to the grace of Christ, even though in substance it is of the natural order. Morally good works done through such help, even by unbelievers, in some way dispose men to justification and so to eternal salvation, and they serve to obtain justifying grace.
'Christ died,' he says, 'that the ungodly might be justified, and also that they might live rightly. For by good works and an upright life, even in moral virtue, we obtain from God the spirit of compunction and of justification.' By this reasoning he thinks it sufficiently shown that this help of grace, distinct from the gift of creation and required for each good moral work, comes through Christ. Disputation 190 already cited, chapter 12.
Latin
XXVI. Unde est quod idem auxilium ad quodlibet bonum opus morale necessarium, licet quoad substantiam sit ordinis naturalis, ad gratiam tamen Christi pertinere contendit. Quoniam videlicet opera moraliter bona, quae per tale auxilium fiunt, etiam ab infidelibus, ad justificationem, & consequenter ad aeternam salutem, quodammodo disponunt, & gratiae justificantis sunt impetratoria. Sicut, inquit, Christus mortuus est, ut impii justificarentur, ita etiam ut recte viverent: nam & bonis operibus & recta vita, etiam secundum virtutem moralem, spiritum compunctionis, & justificationis a Deo impetramus. Qua ratione putat sufficienter probari, hoc auxilium gratiae distinctum a dono creationis requisitum ad singula bona opera moralia esse per Christum. Jam citata disputatione 190. cap. 12.