Regarding the Catholic:  Renouncing the Catholic faith, "repenting" from that system and turning to God would clear him of his decision. Men should always be careful to "make and oath." We can say things rashly with our tongue either to God or man that we may later regret. But God is gracious to forgive when we repent. Jesus taught that we should just let our "yes" be "yes" and our "no, no" (Matt. 5:37). Furthermore, Solomon warned:

"It is a snare for a man to devote rashly something as holy, And afterward to reconsider his vows" (Prov. 20:25).

Ecclesiastes 5:5, "Better not to vow than to vow and not pay."

I would think two passages would weigh heavily on the last part of your question.

"For there are eunuchs who were born thus from their mother’s womb, and there are eunuchs who were made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the kingdom of heaven’s sake. He who is able to accept it, let him accept it." (Matt. 19:12)

The one who chooses to make himself a eunuch for the kingdom of heaven's sake has chosen the hard way. It may be because he had been married and was divorced and has to remain celibate in order to be right with God (Matt. 19:9). But one may also choose to simply devote all of his life in a service to God which would not permit him to have a family. Yet if he was never married and later found a woman that he wanted to be as a companion in the faith then God would surely permit it by this second passage:

"Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Heb. 13:4).

God has given the honorable right of marriage to everyone and a man is a fool, in my mind, to vow that he will abstain from this right for his entire life. I am not saying that he is a fool to refrain from marriage; I am saying that it seems foolish to "vow" such. If anything, in your scenario, he should repent to God for making such a vow to begin with. Now, those who have been divorced and who are not given the right of remarriage could vow but surely commit to "being a eunuch," so to speak, in order to remain faithful to the Lord's command (Matt. 19:9).

Also concerning vows, under the OT, a woman's vow could be overruled by her husband or father (Num. 30). Wouldn't Christ have more authority than an earthly father to release us from an oath if we truly repent of it? Some vows are simply sinful to make in the first place and going through with them is adding sin to sin. For example, think about Herod's foolish oath of giving Herodias' daughter whatever she wished after she sensually danced before him. When she wanted John's head, Herod should have repented for speaking rashly rather than adding sin to sin by fulfilling his ungodly vow. To make that oath was sinful to begin with and to fulfill it was even more sinful to end with (cf. Matt. 14:7-9). Likewise, Peter denied Jesus with an oath (Matt. 26:72), obviously, he needed to repent for ever giving the oath to begin with. Some of the Jews made an oath saying that they would neither eat nor drink until they kill Paul (Acts 23:12-14). That was a sinful oath. What if one of them, after making that oath, began to reconsider and listen to the apostle and believe afterwards? Should they fulfill their oath? Obviously not. What if a young widow made an oath to never marry again after her young husband dies, but then later she heard or read where Paul desired the younger widows to marry (1 Tim. 5:14)? Wouldn't she need to reconsider and repent of her oath?

What then about a man who vows to never marry, but then studies that in order to reach his fullest potential in the Christian religion requires one to be married with submissive and faithful children (viz. elder/bishop/pastor or deacon: 1 Timothy 3:1ff; Titus 1:5ff). Should he reconsider his erroneous and ignorant error and repent for ever making his vow to begin with or continue to fulfill it? When a man makes a vow to "never get married" he makes a vow to "never become a shepherd" in the Lord's church. Who is more pleased with such a vow, God or Satan? God is certainly gracious and willing to forgive us our sins.

"If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (1 Jn. 1:9).

The only sin mentioned in scripture where one can never receive forgiveness is blasphemy of the Holy Spirit (Matt. 12:31). That subject is another issue altogether.

Consider also that:

• Preaching to forbid marriage is satanic (1 Tim. 4:1-3).
• God created marriage for man's benefit. "It is not good for man to be alone" (Gen. 2:18). Why would one purposefully take a vow to "be alone" then. (The subject of remarriage is altogether different: Matthew 19:1-9)
• Marriage is also for a society's benefit because its purpose, if people would get back to the scriptures, is to procreate "godly offspring" (Mal. 2:15). It clearly is a fundamental building block for a nation. Divorce, however, spawns violence (Mal. 2:16).

Obviously, we are to fulfill our vows. But some vows are sinful to make and even more sinful to fulfill. A broken vow may make us look as an untrustworthy person and will become a sin to us.

Ecclesiastes 5:4,  "When you make a vow to God, do not delay to pay it; For He has no pleasure in fools. Pay what you have vowed" (NKJV)

Others may not be able to forgive such but God can and will when we repent and confess our sins to him. When we have deep sorrow for our failure and a deep resolve to conform to our Lord in word and deed, the Lord is merciful and forgiving.

Jeremiah 3:12, "Go and proclaim these words toward the north, and say: ‘Return, backsliding Israel,’ says the LORD; ‘I will not cause My anger to fall on you. For I am merciful,’ says the LORD; ‘I will not remain angry forever.' "

Psalm 41:4, "I said, 'LORD, be merciful to me; Heal my soul, for I have sinned against You.' "

Perhaps we should just take James warning at face value (5:12):

"But above all, my brethren, do not swear, either by heaven or by earth or with any other oath. But let your 'Yes,' be 'Yes,' and your 'No,' 'No,' lest you fall into judgment.

People who speak flippantly and employ the name of God to swear a vow make themselves slide into the pit of judgment. Judgment by others as well as the judgment by God. So, we ought to be content to just speak plain and do all we can to be a people of our word.

Sidenote:  Paul did call God as his witness, but these were in reference to things which he had done, not so much to things that he would do (2 Cor. 1:23; 1 Thess. 2:5, 10; Rom. 1:9; Gal. 1:20; Phil. 1:8). It is usually foolish to vow to do something in the future seeing that the future is not yours to promise it to anyone (Jas. 4:14, 15). Thus, even in the marriage vow, it is stated "till death do us part."