Bear in mind the Vraks books (5,6 and 7) are focused upon Death Korps of Krieg fighting renegade
IG, so the focus is on those forces rather than the Red Scorpions. The various marine chapters who turn up play bit parts, and get their asses handed to them, before moving aside for another chapter.
Book 6 has a small section on the Red Scorpions. The only difference to a standard setup is that sergeants can be upgraded to an apothecary for +25pts, with the downside that no Red Scorpion units in the game are allowed to scout or infiltrate (the old 'have pride in your colours' rule from the 4th edition codex). If you buy this book just for Red Scorpion info, it will be a bloody expensive price per page!
Book 7 of Vraks features the 'Red Hunters' (what a completely uninspiring name for a featured chapter,
) and Grey Knights, but no Red Scorpions.
Annoyingly book 4 is the only one I don't have, however the Red Scorpions are the featured chapter (including the special character that
FW have released, the guy with the mohawk, is it Commander Kuln?) On this basis it is probably the book with the most Red Scorpion info on it, although it is the only one I didn't buy on the grounds I had a quick look through it and the story seemed a bit thin (and the book itself smaller) than the other ones.