There are multiple ways to increase a Pokemons happiness, or friendship. The two are one and the same. for bw2 these include
-having the Pokemon hold a soothe bell
-walking with the Pokemon in you party
-getting it a massage from the woman in castelia city
-using vitamins, such as iron, protein and carbos, on the Pokemon
-leveling up the Pokemon
There are also things that lower a Pokemons happiness, including
-fainting
-use of herbs purchased from the market in drift veil city
If you do and don't do the things mentioned above vinegary should eventually become happy and evolve into lopunny upon leveling up.
This link should help:
http://bulbapedia.bulbagarden.net/wiki/Friendship