For all events I would recommend a rail shirt or jacket ( if you don't know what this is just look on google images)
Personally I find vests a bit dated looking, but that's my opinion. If you are showing at a rated show in country or single pleasure driving some people do wear an evening gown type dress. If you decide to do this make sure it comes to your ankles and that there won't be any wardrobe malfunction. For liberty people tend to be more relaxed since it is at the end or late at night, but normal show attire is fine.
For showmanship you can wear a jacket or rail shirt, some people have the whole color coordinating outfit-- top, pants and boots.
Have a good fitting hat, tailored pants, belt, and boots ( a must for all outfits).