You would need to say which "retro" ideally. That last one you've posted is very '60s, the middle one I would say more '40s, the first one kind of '50s and a bit more American style I'd say. So quite a bit of difference and most vintage fans I know have a particular era they prefer. If you have examples of things she already owns, or type of hair she likes for example that would help.
You could also look at Little Wings Factory and/or Lindy Bop which have lovely dresses in a range of styles from '20s to at least '50s.
Or how about getting her a voucher so she can choose herself? I think I'd actually prefer that if it was me!