I'd check out Smartsound, it seems to be a lot easier and more user friendly for scoring. Sorry SoFo :)

Vegas doesn't have any of ACID's music composition tools. For example, there's no beat mapper, no tempo matching, and the nice semitone pitch shifting of ACID isn't really there in Vegas. If you're used to composing in ACID at all i think you'll find Vegas to be frustratingly limited in this area. Using both ACID and Vegas side by side is probably the best solution. For that matter, if the video portion of your project is more or less complete, you can import that into ACID Pro and score the video in ACID so that you can easily line up the music with the action.

I had a short film in some festivals last year, in which we ran across a copyright problem and had to substitute all the music asap. I had little old Acid Music 3.0, and used loops and a short recording of a friend on electric guitar, and composed three songs to replace the original music. Saved the projects as wav files, burned them onto a CD, took them to a post house (DuArt labs in NYC) to replace on the music track, and the professional engineer there (1) required no tweaking at all in syncing up the music, and (2) tapped his foot playing the music off the studio speakers!

Moral of that story, in my opinion, is (1) review a bunch of music you like first, to find the mood you're looking for; (2) roughtly match the tempo of an Acid project to the tempo you like; (3) lay down rhythm, then bass (unless the music is more classical, or new-agey, etc.); and (4) layer on some loops until they feel right. THEN worry about sync'ing them into the video track.

P.S. Over time I had accumulated a pretty large loop library of about 15 CDs, but you can narrow down the collection you want to work with by, as advised above, screening some regular CDs first and making an initial decision on what style you want. Each film is going to be a different quest.

If you just want to assemble music parts on the spot, with loops that you just stick together, you can consider Acid Music (and don't forget to download the 8-packs every week!).l
Acid Pro is far more powerful, you can *create* your own loops, match tempos or video right inside the program, etc... but is it really what you need?
You can do a lot with Acid Music then just match the lenght to fit clips (in Vegas) if it's not too radical to the point of altering sound quality. And if it is, you just change the tempo in Acid, render again and you're there, ready to try again in Vegas (you will probably get used to "hit on the first strike" then).

Key word here: your NEEDS...

Ok, I checked out smartsound, and there a two versions. Are they both compatable with Vegas?

Emmo

Both are compatible with Vegas but the cheap one only allows you to use 22KHz files while the expensive one allows you to use 44Kh. Either will work with Vegas but the latter is better quality. (By compatible I mean that you compose the music in Smartsound, then save the compostition and then bring it into Vegas).