Hey, guys.

I'm looking for some Progressive Sludge that isn't too heavy, but quite melodic and tranquil with Post-Rock like riffs and with a slow tempo. Clean vocals isn't exactly a must, but I guess a good combination of both harsh and clean is good enough.

The best I could think of right now is Cult of Luna's Crossing Over, Giant Squid's Summit and Irepress (Although Irepress has too much emphasis on Technicality. Less technical I guess)

Thanks a lot.

Try The Ocean, there's some songs with only clean vocals, especially at their newer works.

http://www.youtube.com/watch?v=QvDXmeqNDBo

The newer Mastodon albums.

Neurosis would be right up your alley. They're pretty much responsible for Cult Of Luna actually existing.
You should listen to A Sun That Never Sets and The Eye Of Every Storm first. These albums have almost entirely clean vocals. Everything before and after is much heavier, both on the vocals and on the music.

If you don't mind most of the vocals being screamed, then the "Post-metal" genre would be perfect for you. Neurosis pioneered the genre and Cult Of Luna play it as well. It is basically post-rock played by sludge metal bands.

The best and most popular bands in the genre are definitely Neurosis (albeit they are very experimental), Cult Of Luna, ISIS, Callisto, Pelican (instrumental) and Rosetta. All of these bands are excellent.
If you ever need more post-metal insight, send me a message, I've got a ton of albums to recommend.