"virtue is found in the middle.
Courage is the virtue between recklessness and cowardice. Hope is the virtue between presumption and despair. Humility is the virtue between vanity and self-deprecation."
I don't think so. I think it appears that way, because we are seeing a 3-dimensional thing with 2 dimensional eyes, so it appears #3 is sitting between 1 and 2, when really, its a triangle, and 1 and 2 are only a few feet apart, while 3 is actually a mile away from them. (does that make sense?)
At least, all my life, I struggled with what humility was. Like you said, it seemed this delicate balance between self-deprecation and vanity. There was something so...creepy/sad/irritating about self deprecation. And I wondered how you balanced it just right etc etc, until something CS Lewis said really hit home to me. Both Self-Deprecation and Vanity are focussing on self, whether to extol or harangue. But both are looking at yourself.
And as a teen, an older girl told me "Humility is truth, it is seeing yourself as you really are". And suddenly it made sense. Humility isn't this balance between the self-haters and the self-lovers, figuring out the right balance of censure and praise to give yourself.
Humility is knowing yourself as you really are---and fixing your eyes on God. Humility is its not about you. You don't look at yourself, to revel in loathing or pride. You're too busy looking at God.
Same with Hope. I don't think hope is in between presumption and despair. Both Presumption and Despair are God-not-being-in-the-picture. Despair, obviously. (I mean, w/o God, looking at the horrors in this world, it does make sense to end it all, like that creepy-planet-euthenasia thing in the end of Season 4 of Doctor Who) But presumption also is not looking at God. The faith in ourselves, in cheerfulness, etc to make things right. That's presumption.
But Hope is looking at God. With him, we see, yes the world really really really sucks and evil is more horrific than we imagined, and yes, our frail attempts at fixing it can't do much--but there's HIM. And we can not presume too much good/hope about God. He does "more than we can ask or think". He's God.
So virtue, isn't in the middle. Often both "poles" or "sins" are actually, at teh root, the same (self-deprecation and vanity = focussing on self; despair and presumption=not putting God into the picture) and virtue, at its root, is looking to God. Fixing our eyes on Him, expecting, waiting, worshipping, seeing Him for who He really is. "Christ, our Hope" and all that.
Virtue does not fall between 2 vices, but rather, its on a whole new dimension.