If I'm typing a URL out, I'll generally remember the name of the product but not the actual .xxx, so I'll assume .com. But I imagine Vine doesn't really drive much of its interaction through people typing out the URL.

Vine is used primarily from within apps. If anything, having a .co is a slight helper as their primary target is Twitter where characters are often a premium.

I don't mean this as opposition to you, tmmm, merely that you got me thinking on why one might prefer a .co or be fine without a .com.