I must say, that while I am one to experiment (to a certain extent) with my personal style, I rarely wear vests. Not because I dislike them. They actually add a little je ne sais quoi to your look if you do it right. But how do you wear a vest as a woman without looking awkward or like you're trying too hard? One word: texture.
Take the StyleMint vest I'm wearing for instance. It's black, but because it's this shaggy texture, it's more interesting. It pops against that Kelly green shirt and gives this super casual look some character. A lot times, when we want to take our look to the next level, we automatically assume we MUST throw in color. While that's true to a certain extent, texture goes a long way. So does shape. Something more structured with menswear appeal or a vest with oversized lapels can give you "it girl" appeal.
To be honest, that's really how you can take inexpensive clothes and make them appear more expensive. Texture and shape. But whether you're wearing fast fashion or a high end designer, you should always dress to express (yourself) first before you dress to impress (other people). You always come first ;)