I cannot open the book I downloaded from Digital Editions because my error message says: The vendor account you entered is not associated with the item you are trying to open. Try again. I know my Adobe ID and password are correct. Any suggestions?

Try this (it worked for me):

1) In ADEditions, go to Help --> Erase Authorization

2) In ADEditions, go to Help --> Authorize [using Adobe ID]

3) Select Book(s) --> right click --> Remove from Library

4) Re-add book(s) (in case that you have an .acsm file with a link, open this file with ADEditions)