Hi Susie! Vanilla is a little bit infamous for turning soaps brown, so it’s nothing you did wrong at all. :)
Unfortunately, it’s the cost of having a vanilla scent.
However, you could try adding a little zinc oxide to your soaps to help lighten them up a bit, or you could embrace the dark coloring and maybe make a swirl or striped design with half of the soap unscented (so it will be white) and the other half scented (so it will be brown.)
I do that with honey, since it also turns soap golden brown & just make it part of the design feature. :)