I think that they don't want to be traced If you remember, Dan and Amy should have been hostages as well but they escaped. The Vespers probably wanted to do it by themselves and stop the Cahill family from interfering but Dan and Amy's escape prevented that.
But if they knew where everything was then why not just steal it right away? What are the reasons for having hostages? It alerts the Cahills! If they didn't want any Cahill interferance then they would just steal it right away without getting the Cahills involved.
And why can't the Madrigals send more experienced agents other than just Amy and Dan.
And how come only characters from the Clue Hunt series 1 were kidnapped (with the exception of Phoenix)
Because the vespers are lazy idiots who are pure evil who have never heard the word "love"in their lives!!!!!!