[giancarlostoro]

Vine was married to Twitter for login / registration. It made the most sense for Twitter to buy it. Microsoft's hitting a momentum of growth currently, which might be a good time for them to make some acquisitions, they've also started booting back up some of their game studios as well.

[someperson]

Grandfathering existing user's Twitter login then having all new users use a new login system would not have been hard.