As a preface, the original source of the claim is Arthur T. Vanderbilt II in his 1989 book Fortune's children: the fall of the house of Vanderbilt, on page ix of the introduction. (Quote also mentioned in this 10 September 1989 review of his book.)
Can the occurrence of this 1973 family gathering be confirmed?
Yes, according to Tennessee law:
WHEREAS , Vanderbilt University will begin a celebration of its Centennial on March 16– 17 , 1973 with a reunion on campus of the Vanderbilt family which has graciously assisted in the support of the University for six generations
See also the 19 March 1973 New York Times article All Those Vanderbilts Hold a Family Reunion which explains that only 80 descendants (plus 40 spouses) out of the 592 living descendants attended.
Gloria Vanderbilt and Carter Burden, two of the better known New Yorkers in the family, did not attend.
Some of the Tennesseeans at the celebration were surprised to learn that not all Vanderbilts are extremely wealthy. The Commodore's millions have been diluted in some branches of the family.
“Some of us work for a living,” Peter O. Wilde of Cambridge, Mass., said at one of the cocktail parties. He is a real estate consultant and a teacher at Harvard Business School.
Is it accurate that none of the attendees were millionaires at the time?
The article says that Duke of Marlborough John George Vanderbilt Henry Spencer Churchill was at the reunion. He was very wealthy according to the Wikipedia article.
Were there any millionaires among Vanderbilt's descendants in 1973 who did not attend this reunion?
Yes, Carter Burden was a descendent who, as explained above, did not attend, but was a millionaire according to the 7 November 1971 article The Carter Burden question—Can a rich, handsome young member of the Jet Set from Dry Dock Country find happiness as a reform machine pol?:
millionaire owner of The Village Voice