"Viva Las Vegas" and "Roustabout" were probably about the cutoff point in Elvis' soundtrack career (and movie career) where things just started heading downhill. In "Viva Las Vegas", you have the classic title track that everyone who is everyone knows from every movie that ever has anything to do with Vegas these days -- they all find a way to get "Viva Las Vegas" in there somewhere and it is a great song. Other solid songs include "What I'd Say" and "C'mon Everybody", two rockers, and the ballad "Today, Tomorrow And Forever". The rest are so-so, but have some decent vocals from Elvis, along with a couple of duets from Ann Margaret, which really spices it up, especially "The Lady Loves Me", a playful little tune.
On Roustabout, the good songs include "Roustabout", "Little Egypt", "Big Love Big Heartache", "One Track Heart". You can even really get into "There's A Brand New Day On The Horizon", "Hard Knocks" and "Poison Ivy League" -- all nice little up-tempo songs that are fun.
Overall, I'd say this is where Elvis really put in his last true inspired Hollywood soundtracks (and acting, for that point) and you can tell he just basically went through the motions from here on out for the next five years. It didn't help that around the time "Roustabout" was released in 1964, the British Invasion was taking the U.S. by storm and basically bumped Elvis from his superstardom monopoly, which probably didn't set well either, and he didn't get back into the groove and become relevant in the music industry again until his 1968 Comeback Special.