IMHO, I think that is because no one uses the Vine app to consume vine content, so they must post to twitter to share their videos. However, with Instagram, I open up that app specifically to look at photos that people I follow in Instagram have shared.

My Instagram social network is largely independent of the other large ones (twitter, FB, G+) with only a slight overlap, so it definitely deserves its own consumption, and would also explain the reason that the vine shares are more prevalent on twitter than instagram shares.