The entry on Alexandre-Théophile Vandermonde at the MacTutor History of Mathematics archive ends with the description of the contents of Vandermonde's fourth and last mathematical paper, concluding with the sentence
Finally he gave a remarkably clever notation for determinants which has not survived.
Sadly, it does not tell us what this remarkably clever notation was! I've searched a bit for the actual paper, Mémoire sur l'élimination, with no success.
Maybe someone knows what this notation was?