Is publishing with SpringerBriefs vanity publication for a beginner researcher? I am a beginning researcher and am unsure as to the academic worth/usefulness for my career to publish with SpringerBriefs. Would it be seen as a vanity publication?
Think "short book", not "long paper". There's been several questions here about the value of publishing a book (e.g., Is there any value in self-publishing a book as an academic? or Pros and cons of (co-)authoring a reference book in early career?), and the answers apply here as well. In short, the fact of having published a book carries virtually no weight (at least in most fields), but a good book can have a long-term impact. 

Original research should in general be published in a peer-reviewed journal (or conference), not a book. But if you've written a few papers on a topic that you think would benefit from a consistent notation, cleaned-up presentation, extended introduction and literature review; or if you have a set of lecture notes on a current hot topic you are particularly proud of and would like to see get wider attention (preferring to trust the power of Springers marketing department instead of the vagaries of Google), then publishing them as a SpringerBrief could make sense for you. Whether it'd be worth the effort to get them into shape (and it will cost effort) is something you have to decide for yourself.