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.
As for the peer review, I talked with a Springer representative at the 2016 Joint Math Meeting and she clarified as follows: two rounds of peer review, once when an author indicates a desire to publish a Springer Briefs (signs the paperwork, submits a table of contents and a couple chapters), and once when the work is completed.
Its not entirely clear from its website, but it appears that SpringerBriefs is not peer reviewed. At least, that's what I get from their author instructions starting with:
Springer Briefs are designed to get your ideas to market as fast as possible. With this aim in mind, we have outlined simple instructions for manuscript formatting, preparation, and delivery. After you have delivered your manuscript to editorial and it is transmitted to our production department, the manuscript will be assigned to one of our full-service production vendors (FSVs).
Given that, my assessment is that publication in this venue is likely to be essentially worthless from a scientific career point of view: it's not peer reviewed, so it's not giving you any more credit than depositing the same text in a repository like arXiv, and it gets you much less exposure than something like arXiv because people have to buy it to read it.