Instead of vows, I'm planning on surprising my bride-to-be with an acoustic love song at the wedding?

I think it will be a very memorable and lovely moment of the wedding, but it should not replace vows. The vows are the primary point of the wedding ceremony, and really shouldn't be replaced by a song. The vows are your promises to one another. I'd love to see that at a wedding as soon as the couple was pronounced husband and wife.