I am not trying to suggest non tznius or clingy styles from the non Jewish stores. But many sleeveless tops can be worn as a vest and sun-dresses as jumpers. I’m not talking about an inherently non tznius style, but it looks like a vest – not low cut or strap sleeves or things like that.
Obviously the shell underneath must not have tight sleeves and the jumper/vest must fit properly. Children’s Place sells jumpers/sun dresses in the summer – that look perfect as a jumper with no tznius issues whatsoever!!!