Quite possibly, yes. The Vanderbilt family probably owned diamonds and probably had them set in engagement rings in the 1920s.
Sight unseen, and without a paper trail -- provenance -- it is impossible to determine whether a diamond is a Vanderbilt Diamond or not, regardless of the age of the ring.
All diamonds in the grand sense, are antique. You may be referring to a cut of stone that was favoured in the past. Commonly, an older cut called a 'mine cut' can be found on stones in circulation today. You can read more, below.
Antique dealers
Vanderbilt's head is called the Chancellor, and the current one is Chancellor Nick Zeppos as of 2008
Diamonds made in factories are called man-made diamonds.
Essentially, diamonds are rocks. They are also called ice.
Loose diamonds are called loose diamonds. When set as a group, the term may describe the setting more than the stones, i.e., pave, which is a 'pavement' of diamond stones. Otherwise multiple diamonds are simply called diamonds.
Coal has been called 'black diamonds'. This is probably because coal and diamonds are both formed from carbon.
They are called blood diamonds because the money made from them is used to fund wars and terrorist activities.
Diamonds are VALUABLE
Man-made diamonds are usually called man-made diamonds.
In Connecticut USA, 25 years or older can be registered as an antique.
an Heirloom