You want them to provide a chain of title for everything including samples. Have them sign a combined master use and sync agreement. In that agreement they warrant to you that they have the full right, power and authority to enter into the agreement, they indemnify you, etc.however some artists have samples in their music therefore do they have permission to give the rights to me? Must they clear the sample rights first or is it fair use?
in this case, the original vocal stems were released to the public by the record label for remix artists to work with as they please. These days in the music industry it's actually pretty common practice, as remixes are becoming more and more popular.