Verified users will have their profile usernames automatically reserved. If you want something different, you can request it, as long as it doesn’t conflict with another verified user’s account. Choose carefully, as Twitter says these URLs cannot be changed.
Those of you without a blue checkmark will need to wait until Monday, December 23 to get your custom link. Keep in mind, though, that unverified accounts won’t have their username URLs reserved, so you’ll need to move fast when they do open up.
Twitter didn’t say when it will launch Vine Web profiles, but it is nice to see the service expanding beyond mobile.