Vegas will come to regret that deal in due time. Pietrangelo was still largely good, but he's already showing signs that his best years are behind him.
That said, St. Louis may have been okay with the whole Pietrangelo/Krug swap, but what really killed them defensively was acquiring Marco Scandella, whose game promptly fell off a cliff not long after he arrived.
They went out and got Faulk + Krug, realized they didn't have anyone that could play an ounce of defense besides Parayko, and ended up with Scandella.
That Blues team is now a regular season team. Sure they'll make the playoffs the next few years, but come playoff time 99% of teams would rather pay an extra $2 mil for Pietrangelo over Krug.