I am also loving what I call a "summer vest," which is basically an open vest (which in some cases may have a tie closure) that is of a modal or jersey fabric. This is meant to be worn with a summery dress or over a tee/tank to create a fun depth to an otherwise simple outfit. I would wear a vest like this over a little cap-sleeved tee, with a pair of lighter-wash jeans and some sandals. I'd add some bangles and some large earrings to accessorize. Or, I'd find a fun, floral sundress and wear the vest over that, pairing it with a nice pair of vintage-looking boots or sandals.
Below are a few of my current favorites:
Linen Fashion Vest ($17.80)/H81 Linen & Crochet Vest ($19.50)
(which I think follows many of the same rules as the Racerback)
BDG Halter Vest ($48.00)