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Have you looked at Sony’s Cinescore? I have it and use it. It itegrates well with Vegas and comes with quite a selection of music when you buy the program. It also has additional music packs you can buy. You can even download individual pieces from those packs. The software allows you to tweak the whole piece and adjust for time.
They have a complete tutorial of Cinescore on their website that shows you how to use it. It should provide enough information for you to decide if it will do what your are trying accomplish.