I'm pretty sure that she didn't feel the need to "come out" or make a big announcement before. She said it in passing at a show for gay pride to share the love, It's not like she sold out to a magazine or something. I don't wish to presume I know Vanessa, but I can't see her doing this for publicity. She usually shies away from fame. Plus her album won't be out for a while, so what's the point? V isn't really a "celebrity", she's a musician, so why should she "come out"? Who's business is it? It was bound to end up on a few gossip sites but it probably just seemed appropriate to say it during her set. It's just a part of who she is that never needed attention before.
And I don't see why she would feel the need to change the gender references in her songs for anyone. I'm sure if a specific song was about a woman, she wouldn't hide that but, like bisexual girls I know, maybe she has more loving and meaningful relationships with men and not as many with women? When we listen to music, we take what we relate to and apply our own take anyway. It's not up to the songwriter to cater for every listener, because that just wouldn't be possible. She's better being true to herself and writing from her own experiences and how she feels. It's worked well so far!
Love you, Vanessa.