I've both a Vivi Nova and a Mini Vivi, and exactly the same thing with both. As PepNYC says, is not enough to be a problem, just enough to annoy me . I hope somebody will come with a way to fix it without breaking the thread...
My Vision Vivi Novas, both the standard size and the minis, fit level on both of my VAMOs. I'm trying some Boge Viva Novas and they fit straight as well. If your Viva Novas fit straight on other devices you might want to consider asking your seller to exchange the VAMO you have.