I don't know about "Dropping Fast" - it's slowly dropping away, even after they changed how they gathered statistics to only count devices visiting the android store.
And really, I'm not entirely shocked that a Twitter-owned service is getting more sharing on Twitter compared with a service that gets a reasonable amount of obstruction from Twitter.
That doesn't mean people will riot if it isn't available on their device. If you've got Android 2.x and can't get Vine, you likely don't really care as you haven't ever used Vine.
Vine only supports 4.0 and above & there's no uproar. It is a Top 3 app on Google Play.