Barneys at the Palazzo. Oh how I love you so. Conveniently near the Venetian and Bellagio where I frequently stay, this place is deadly. Open till 12am, right on the strip, this place begs me to spend my casino winnings there.
Barneys here is a smaller location, with one floor dedicated to mens, and the rest for womens. Very helpful staff, but their selection is a bit limited. You won't find everything you'd see at a flagship store, but hey, you're in Vegas, and you really want, there's a boutique shop for every brand possible on the strip.
Last time I was there, I came right in the middle of a huge sale, so everything was on sale. You'll find brands like Rag & Bone, Philip Lim, The Row, etc. If you like buying clothing, then Barneys on your hit list.