We use v15 and lot's of users forget to select the correct version. So here are some suggestions.
1) Support something like this for a v15 specific login page https://sc2.omniture.com/login/v15/
2) Then allow for the admin to set this on a company wide level. So when new users are created and we send an e-mail to them they get the v15 link not the general http://my.omniture.com link in the welcome e-mail.
This may help a little bit: https://sc2.omniture.com/login/?company=mycompany&u=mylogin&version_options=15
Edit (or skip) query params as you need. This link also redirects directly to SC interface if the user is logged in already.
Works great for me.