Trees show up throughout the Old Testament, and almost always in reference to an encounter with a god or God. To wit:
You shall not plant a tree as an Asherah (Deut. 16:21); Ahaz sacrificed under every green tree (2 Kgs. 16:4; 2 Chr. 28:4); they sacrificed at every leafy tree (Ezek. 20:28); they made Asherim under every green tree (2 Kgs. 17:10); altars under every green tree (Ezek. 6:13); Judah committed adultery with stones and trees (Jer. 3:9); Israel was a harlot on every high hill and under every green tree (Jer. 2:20; Jer. 3:6); you have scattered your favours among strangers under every green tree (Jer. 3:13); destroy worship places under every green tree (Deut. 12:2); their Asherim by green trees on the high hills (Jer. 17:2).
Collin's Thesarus of the Bible, "Trees"
The Asherah Poles, for example, were always associated with deities at best and idol worship at worst. But, the Oaks at Mamre mentioned in Genesis, for example, mark where Abraham met with angels and with the True God himself.
The point then is that it is not the Tree itself but what it represents. If you want to make the argument that Christmas is the Pagan holiday of Saturnalia, and that the tree is nothing more than a manifestation of it, then, yes, you could say that a Christmas Tree is an idol. (Jon Acuff will rightly say you are being "that guy", but I digress.) But, you can also say that for a Christian, the tree is representation of the manifestation of Christ - not unlike the Oaks at Mamre.
As such, if you want to call it a vanity, I suppose you can. Just don't assume you are right in all cases.