ACID loops will retain their tempo information and key information inside of Vegas 5 natively. However, Sony makes ACID, which is considered the 'grandaddy' of all looping software, and it works well for creating projects for working within Sony Vegas software.
You can download lots of free loop samples at www.acidplanet.com
you might also want to visit HarkProductions.com and check out their loops. Their Beautiful Pianos is my favorite loop package from anyone, overall.