What is the role of the Holy Spirit in the Thomistic understanding of vocation?

St. Thomas Aquinas discusses the role of the Holy Spirit in leading one into religious life in his Contra doctrinam retrahentium a religione (Refutation of the Pernicious Teaching of Those Who Would Deter Men from Entering Religious Life) ch. 9, "Answers to the Arguments Used to Prove That Before Entering Religious Life a Man Ought to Deliberate for a Long Time, and Take Counsel of Many":

[…] there is another way [besides Holy Scripture] whereby God speaks interiorly to men, viz. the way alluded to in Ps. 84:9: “I will hear what the Lord God will speak in me.” Now this interior voice is to be preferred to any external speech. St. Gregory says (Homil. Pentecostes):

The Creator does not speak to the understanding of a man, unless He speak to that same man by the unction of the Holy Spirit. Before Cain slew his brother he heard a voice saying, “You have sinned; stop.” But as, on account of his sin, he was admonished by a voice alone, and not by the unction of the Spirit, he was indeed able to hear the word of God, but refused to obey it.

If then we are bound to obey immediately the audible voice of our Creator, how much more ought we not, unhesitatingly and unresistingly, to obey the interior whisper, whereby the Holy Spirit changes the heart of man. Hence, in Is 50:5, it is said by the mouth of the Prophet, or rather of Christ Himself: “The Lord God has opened my ear (i.e. by interior inspiration), and I do not resist. I have not gone back.” “Forgetting the things that are behind, and stretching forth myself to those that are before,” as we read in the Epistle to the Philippians (3:13). St. Paul, again, says (Rom 8:14), “Whoever are led by the Spirit of God, they are the sons of God.” The Gloss of St. Augustine has the following comment: “Such men do not perform anything, but act under the impulse of grace.” But he who resists or hesitates, does not act by the impulse of the Holy Spirit.

It is then the distinguishing mark of the sons of to be carried forward by grace to better things, without waiting for counsel. This impulse of grace is alluded to in the Prophet Isaiah (59:59), “When he comes as a violent stream, which the spirit of the Lord drives on.” St. Paul teaches us that this impulse of grace is to be obeyed: “Walk in the Spirit” (Gal 5:16), and again (5:25), “If you are led by the Spirit, you are not under the law.” St. Stephen thus reproached certain men, “You always resist the Holy Spirit” (Acts 7:51). St. Paul says (1 Thes 5:19), “Do not extinguish the Spirit.” On which words, the Gloss comments: “If the Holy Spirit should at any time reveal something to a certain man, do not forbid him to make known what he has heard.” Now, the Holy Spirit gives His revelations not only by teaching man what he ought to speak, but by suggesting to him what he ought to do (John 14). When, therefore, a man is inspired by this Holy Spirit to enter religious life, it is his duty to follow the inspiration at once, without waiting to take counsel of human advisers. This is shown us by the words of the Prophet Ezekiel (1:20), “Wherever the spirit went, they went, and the wheels rose along with them.”

[…]

Interior inspiration has efficacy to enable those to whom it is given to accomplish great deeds. We read in the Acts of the Apostles [2:11] that when the disciples were gathered together, the Holy Spirit came upon them and made them speak of the wonders of God. The Gloss says on this passage: “The grace of the Holy Spirit of God knows no obstacles.” Again, the Book of Sirach (11:23) has these words: “It is easy in the eyes of God suddenly to make a poor man rich.” St. Augustine speaks of the efficacy of internal inspiration in his book De praedestinatione Sanctorum [ch. 13]. He quotes the words recorded in St. John (6[:45]), “Everyone who has heard of the Father and has learned, comes to Me.” On these words he says, “This school where the Father is heard and teaches, in order that men may come to the Son, is far removed from the senses; for in it we hear not with ears of flesh, but with the hearing of the heart.” Again he says: “The grace which, by the Divine munificence, is secretly bestowed is not rejected by any hard heart; for it is given in order that hardness may be entirely taken away.” St. Gregory, in his Homilia Pentecostes, treats of the efficacy of interior inspiration: “Oh, how cunning a workman is this Spirit. He makes no delay in teaching what He wishes. As soon as He touches the heart, He teaches it. His touch is teaching. He changes the human mind as soon as He enlightens it; and the man taught by Him at once forsakes what he was, in order to show what he was not.” He who hesitates to obey the impulse of the Holy Spirit for the sake of taking counsel either knows not this impulse, or else resists it.