Gladys Knight appeared after turning down VH1 repeatedly since 1999. Patti LaBelle appeared after turning down Vh1 in 1999 and 2003.
This was the last Divas concert, and Vh1 managed to grab many of the big names that had turned them down before, with the exceptions of Barbra Streisand (invited in '98), Lauryn Hill (invited in '98), TLC (cancelled in '04) Christina Aguilera (asked in '00), Jennifer Lopez (invited in '02), Britney Spears (invited in '02), and Kylie Minogue (cancelled in '04).
Gladys Knight only signed on after good friends Patti LaBelle and Cyndi Lauper did, and the producers had to work out a plan for her to perform on the live show in Vegas between her performances on her own Vegas show. They succeeded.