In reply to gizmo: If you are willing to pay decent money you have no shortage of good options from loads of top manufacturers.
However, a synthetic vest is probably one of the simplest bit of gear possible so I do think it worth looking around for bargains including places like TKMaxx or Outlet Shopping Centres. Also, I wouldn't discount own brand versions from the likes of Decathlon, GoOutdoors and even Mountain Wharehouse.
I picked up one by Tog24 for well under 1/2 RRP at Bicester Village a couple of years ago and have found it perfectly fine.
However, when it comes to packability the key thing is not really the vest itself but finding a perfect size of stuff sac for it. I use one originally from a Rab survival zone bivvy bag and it works brilliantly.