Phrases get automatically updated in default vbulletin (just like templates do also). However, if they are modified, and a language pack does modify the default phrase, then it is not automatically updated - you need to do this manually.
Old "login" phrase:
Code:
Thank you for logging in, $bbuserinfo[username].
New "login" phrase:
Code:
Thank you for logging in, {1}.
So, just make that change and it should be fine.