Safari vests are designed with the second part of the old adventure travel rule in mind: Only take enough to be prepared and keep the really important stuff close at hand.
That is one of the characteristics to keep in mind when choosing but there are some more...
This list below is from my own personal experience and the tips and recommendations stored in the travel reviews at African Safari Journals.
1) Pockets for storage.
Twenty pockets is more than enough for most travel situations. More than that and you run the risk of feeling like a pack mule and forgetting where you stowed what.
Gets frustrating fast having to search through multiple compartments each time you want to find something.
A few interior pockets are also a good idea for the really valuable stuff.
2) Colour and material.
If you are going to be walking in the bush you need to be as inconspicuous as possible (inside a vehicle colour doesn't matter to the wildlife) so the only practical colours are khaki, olive drab, tan or charcoal.
Stay away from bright colours, black, blue, white and camouflage patterns.
You are going to be spending a lot of time exposed to the sun so you will overheat if your garment doesn't "breathe" properly. Cotton fabric, selected synthetics and mesh design is ideal.
3) Weight and washability.
If you pack anything like me then suitcase space is always a problem so it helps if you have a lightweight garment that is not very bulky. Another option is to wear the vest when you board the flight, clearing that space in your baggage for other things.
You can get almost as much carry-on gear into the pockets as you can into a travel bag.
Dry cleaning facilities are few and far between in wildlife areas (unless you are at a luxury lodge) so a hand or machine washable garment is an advantage.
If you are on a short safari then there probably won't be a need to wash it until you get back.
4) Length of the garment.
It may not be evident at first glance but the length of safari vests also make a difference. Too long and you can't sit down comfortably when there are heavy items in the bottom pockets.
Too short and there might not be enough space for everything you want to carry, so make sure that the length is right for your particular situation.
5) Where to buy.
I always start my searches for outdoor gear at Orvis as they give a great idea of what's available in the market at the right price/quality ratio.