Beauty and the Beast is special because it's not a typical love story. It's about a Creature There Has Never Been. Vincent is different. The ongoing attempts of normalizing his relationship with Catherine into a brownstone with access to tunnels, the BatB version of the white picked fence, are testament to how difficult it is to understand this. It's something he had to struggle with all his life. Like all the different people, I think.
I have been noticing that something changed in the perception of the show along the years. In the beginning, it was the magic of Vincent and his tunnel world that the fans mostly fell in love with: his difference, his struggle, were inspiring and he represented a role model for many. The point of view slowly shifted, and now the fans rather identify with Catherine, who becomes a very "normal" Catherine, having normal expectations about a normal love story. And Vincent is perceived as a furry guy with low self-esteem problems who happens to live Below, whom a smart Catherine rescues and normalizes into a furry husband.
Instead, I think that Vincent fell so hopelessly in love with Catherine because he felt that she understood his uniqueness completely, even since that terrible moment when she saw him killing those men in the Pilot, and did not hesitate to take his hands and accept to share the responsibility. The end of his aloneness, he said (Temptation). Unlike Devin, who included Vincent in his dreams, but had to leave him behind to live such dreams, she fully embraced his differences, respected them, and, especially, accepted to be shaped by them.
Together, they are “something that has never been” (Orphans). Their journey “is one that none have ever taken”, and they are “just now setting out”. The uniqueness of Vincent makes this journey symbolic of the journey that any lovers take. They start as individuals who are separate and different, and walk the same path, hand in hand. They remain separate and different, there is no merging in a healthy relationship. Only, all that they are, all that they can ever become, is shaped by their being together. If the same individuals walked that same path alone, they would end up being different from what they end up being together. They are mirror to each other, they help each other to interpret events and situations. If the lovers don't do it - and it's not easy, it means accepting the limits that the other represents - somewhere along the path they go back to being strangers who happen to be walking the same path. Vincent and Catherine are for each other the possibility of being something else, something more, something better. Precisely because they are not trying to “normalize” each other into their limited vision.
Vincent is not asking of Catherine to leave her world, despite his profound wish to have her Below. “I don’t want you to go back”, he said in Orphans. First time he said anything so possessive. But when Catherine understands that it’s not what she really wants, he accepts to set her free and helps her not to consider it a failure. “There is no failure. It doesn’t mean our dream can never be. It just means that now is not the right time”. (Orphans)
Catherine is not asking of Vincent to be what he’s not. He’s the Beast, and although it’s difficult for her, like for everybody, even for Vincent himself, to understand who and what he really is, she is willing to accept her “fate” and to love the “limits” that it involves. Gratefully. (AFAPK) She did dream of sharing icecream walking down 5th Avenue in the sun (TBW), but then she opened her eyes, and shared with him The Idyllis of the King in the shadows, proclaming that the flawed Lancelot is the greatest knight of all. (WTBBS)
Yes, they shape themselves, as a couple, into something that has never been, fostering the possibility of being for each other. Such possibility of being, in my personal opinion, is best perceived as the beautiful borderland where they are for each other a bridge to the "other" world. It's the threshold Below and the threshold Above - Catherine's balcony - where worlds meet, Below meets Above, and Above meets the Sky. Vincent and Catherine represent for each other - and for us - the journey from the innermost depth to the boundless infinity - and vice versa.