But this need to discern a vocation also implies that every true vocation  is a service to God and/or His Church. To quote from Saint Paul,
"But all these things one and the same Spirit worketh, dividing to every one according as he will. For as the body is one, and hath many members; and all the members of the body, whereas they are many, yet are one body, so also is Christ. For in one Spirit were we all baptized into one body, whether Jews or Gentiles, whether bond or free; and in one Spirit we have all been made to drink. For the body also is not one member, but many. If the foot should say, because I am not the hand, I am not of the body; is it therefore not of the body? And if the ear should say, because I am not the eye, I am not of the body; is it therefore not of the body? If the whole body were the eye, where would be the hearing? If the whole were hearing, where would be the smelling? But now God hath set the members every one of them in the body as it hath pleased him. And if they all were one member, where would be the body? But now there are many members indeed, yet one body. And the eye cannot say to the hand: I need not thy help; nor again the head to the feet: I have no need of you. Yea, much more those that seem to be the more feeble members of the body, are more necessary. And such as we think to be the less honourable members of the body, about these we put more abundant honour; and those that are our uncomely parts, have more abundant comeliness. But our comely parts have no need: but God hath tempered the body together, giving to that which wanted the more abundant honour, That there might be no schism in the body; but the members might be mutually careful one for another" (1 Corinthians 12:11-25).That is to say, there are many gifts and talents, and this is because there are many different vocations among the Church. Not everyone is called to be a priest, or a monk, or a nun. And while those may seem to the positions of honor in the Church, the laity are every bit as important to the life of the Church. To quote from the Catechism of the Catholic Church,
"By reason of their special vocation it belongs to the laity to seek the kingdom of God by engaging in temporal affairs and directing them according to God's will. . . . It pertains to them in a special way so to illuminate and order all temporal things with which they are closely associated that these may always be effected and grow according to Christ and maybe to the glory of the Creator and Redeemer" and ""Love is the fundamental and innate vocation of every human being" (CCC 898 and 2392).
Which is to say that there are plenty of lay vocations which fit into the life of the Church, so long as they are performed out of love for both God and Man.
If every true vocation is really necessary for the Church, then that means that every person who enters his vocation and does a good job of it for the glory of God and for the aide of the Church and community is being sanctified through his vocation (by God's grace, of course). Every person who enters a vocation which is his true calling is in effect replying to God's will for his life, so he is also making a sort of prayer of his occupations. He is also therefore receptive of God's grace.
If the Church really does need all vocations, then all vocations come with the possibility of sanctity, provided they are done for God's glory. This means that there must be grace available, not only to priests and religious--the "Fathers" and "Mothers" in the Church--but also to "ordinary" lay mothers and fathers, not to mention to a host of other true callings. That also would imply that people in any of these vocations are called to be saints, not simply by giving up these vocations, but by working to be a saint within everyday life. Thus, for example, in the case of Chesterton, his vocation was to be a husband, a journalist, and a "secular" theologian and "popular" philosopher. In his day as in ours, the Church needed all of these things, and Chesterton embraced these vocations as his own.
That the way in which sanctity is displayed while following these particular callings is different from the way in which a Mother Teresa or a Francis of Assisi made manifest their own sanctity does not take anything away from either Chesterton's own holiness, nor from the Church's own treasury of sanctity. That his life gave testimony in a different way from St Dominic's or Saint Augustine's or Saint Vincent De Paul's or Saint Therese of Lisiux'z does not in the least diminish the witness of any of these saints, but rather complements each and all of them, so that all these different types can bear testimony in his own unique way.
To use an analogy (if one which is a bit cliche), each of these different saints had a different way of giving witness to their faith in, to their hope for, to their love of God, much as a group of blind men might describe an elephant by touch. We are, after all, all of us blind to some extent, seeing at best through a glass darkly (see 1 Corinthians 13:12). That the blind men each describes the elephant differently does not mean they are seeing a different elephant; rather, the elephant's description should be a synthesis of the men's descriptions, though even this will (of course) fall a bit short. The same is true of holiness and sanctity, of the virtues (especially faith, hope, and love), and ultimately of God Himself: the best description we can have comes from a synthesis of the lives of the saints, with a great deal of assistance from revelation (even the blind men working together can't ascertain the elephant's color).
To throw away the blind man's description of the elephant's tail because it does not match the descriptions of the other blind men would be to lose real knowledge about the elephant: the picture would be less complete. Thus, to throw away Chesterton's witness as a saint merely because his life was not the same as a so-called "conventional saint's" is to throw away a little bit of the mystery of holiness , to lose a little information about sanctity and service to the Lord. Indeed, it would be to throw away a little bit of genuine knowledge about God. How tragic that would be.
 I can think of a handful of "false vocations," by which I mean occupations which are not needed by God or the Church: but every one of these vocations involves some form of sin as the primary occupation. Thus, pornographers, prostitutes, abortionists, terrorists, executioners, and other jobs whose purpose is to destroy public morality and/or freedom are false vocations: they do not help the person who chooses these paths to become holy, and put the person against both God and His Church. That is not to say that such people can't find forgiveness or become holy, but a part of this is to leave the sinful occupation behind. Another way of saying this, I suppose, is that no occupations which is sinful by its very nature is a person's true vocation, so a person who chooses such a way of life is therefore also acting against God's will, and therefore rejecting (some or all of) the grace which would otherwise have been made available to that person.
 Again, this general argument can't necessarily be stretched limitlessly. There are a great many people who are not saints, and whose lives tell us little if anything which is true about God or the Faith. That is why there is a long process in canonization, including the necessity for two confirmed miracles attributed to the saint in question. Moreover, not every aspect of even a saint's life is necessarily holy (the saints are, after all, sinners too, and fallible to boot). Thus, for example, the canonization of Saint Francis does not mean by necessity that we all should wear brown sack-cloth robes everywhere we go.