Tom as One of the Valar?
The theory that Tom Bombadil is a Vala is a popular one that's has strength but as we shall see its weaknesses make it near impossible because it is inconsistent with what we know of both Tom and the Valar. The Valar are all accounted for in The Silmarillion (15-21), and of those only one couple can even remotely fit with Tom and Goldberry: Aule and Yavanna. This is indeed the most popular Valar theory. Aule is the chief craftsman of the Vala; he made the dwarves and many of the great works in Middle Earth. Sauron was originally a Maia underneath Aule. Sauron learned much of his craftsmanship (ring making) from Aule. Aule's wife on the other hand is the Vala over everything that grows on Earth which is not a Child of Illuvatar (nature). Yavanna is the one who requests for the Ents to be made to help protect her creation from the Children of Illuvatar. While Goldberry and Yavanna do share some similarities Tom and Aule pose more of a challenge.
One of the strengths of this theory is that it can possibly answer why the Ring has no hold over Tom. As Aule, Tom would be the master craftsmen, and thus he may have power over the Ring. This though assumes that power is the answer to not being under the influence Ring. This theory can also explain the age of Tom and him as “Fatherless” or “Eldest” being that Aule is a Vala. Unfortunately, this is where the strengths end.
The weaknesses of this theory are many and in my estimation irreconcilable. First, Tom is described as nonsensical on several occasions in the Lord of the Rings and this is hardly an apt description of one of the mightiest of the Valar. Second, while this theory adequately answers why Tom is not affected by the Ring it does not answer the reverse side of Tom's relationship with the Ring, his carelessness and disinterest. Surely Aule, the chief craftsman, would recognize the value and importance of the Ring and would never lose the Ring as Gandalf says, “He [Tom] would soon forget it [the Ring], or most likely throw it away. Such things have no hold on his mind” (FOTR, Council of Elrond, 348). Aule is the one who Sauron learned his craftsmanship from; so surely Aule would recognize the importance of the One Ring. The Ring is exactly the type of thing that Aule, the god of craftsmanship, would hold onto in his mind.
Tom's relationship with the Old Forest should also cause pause in accepting this theory. A brief read over the chapter Of Aule and Yavanna will show that Yavanna is indeed close with the forest but Aule is not as evidenced by his character and the character of his children the Dwarves. Tom in opposition to this appears to have a close relationship with the Old Forest.
Another weakness found in this theory is the lack of power Tom would have to resist Sauron the Maia. Surely one of the most powerful of the Valar could resist Sauron, but the Elves say Tom could not defeat Sauron. This statement is made in context of Tom having the Ring and yet somehow Aule, with the Ring, could not defeat Sauron without the Ring? This simply cannot be. Saruman and Gandalf both operate under the belief that they could at least defeat Sauron with the Ring, surely a Vala could do the same. Also, Tom has said his knowledge fails out east, but that would not be true of one of the Ruling Valar.
Even if the argument that Tom as Aule adequately answers him as "Fatherless" it does not answer him as being "first" and as him being the last to fall in Middle Earth if Sauron wins. Of the Valar the first is most definitely Manwe, not Aule.
For these reasons it is simply unthinkable to suggest that Tom is a Valar for he is not powerful enough to be one. Indeed, Tolkien puts the nail in coffin in letter 144 where he writes of Tom, "Ultimately only the victory of the West will allow Bombadil to continue, or even to survive. Nothing would be left for him in the world of Sauron." If the West does not prevail Tom will cease to be, this could not be the case if Tom was Aule. Tom needs the West, but the Valar have withdrawn from Middle Earth to the Undying Lands. The Valar do not need the West to survive, but Tom on the other hand does. It should be noted that even after his defeat, Morgoth still survived, being a Valar, he is just imprisoned not destroyed.
Also, any suggestion that Aule would not take the Ring seriously is pure nonsense. We read in the Book of Unfinished Tales that Aule is the one who chooses Saruman to be sent as one of the Istari to combat Sauron. Aule is thus established to be in the Undying Lands and Aule's care for the plight of Middle Earth and the damage Sauron has caused with his Ring is evident. Aule is very much concerned with the Ring, Tom is not. Moreover, Tom's relationship with the woods does not fit well with the character of Aule the craftsmen. This theory must be acknowledged as fatally flawed in reconciling what we know of Aule, the Valar, and Tom. It would be rather silly to hold to Tom as Aule knowing what we know of Tom’s relationship to the Ring and his potential matchup with the Ringless Sauron. So now that this theory has been carefully considered and found lacking we can move on to consider Tom as one of the Maiar.
 Yavanna cannot be Goldberry for Yvanna is one of the Eight mighty Valar and her reign over plants and wildlife is well known, yet Goldberry we are told is the “Riverwoman’s daughter.” This description does not fit with Yavanna the Vala for she is no daughter of anything.
 For instance, Tom appears in the likeness of a man, yet the Valar are said to appear in the likeness of the Elves.
 Aule is described as one of the Eight of the mightiest Valar whose majesty is unmatched and who rule over the other Valar and Maiar. Tom does not fit well with this description.
 Aule, as stated in footnote 3, is one of the eight ruling Valar and his existence should not be tied to the victory of the West over Sauron.