I am getting some serious inspiration from this photograph:Re-create the look:
Vest from Zara
Slouchy grey long sleeved tee from T by Alexander Wang.

I always tend to wear tighter tops under vests, I like one aspect of my outfit to fit with me (to not make me look bulky). But this really works, so I have got to give it a try! I also love how the muted colours work so brilliantly together.