The only thing that might help is if you can separate the fabric threads into each individual ply.
Each thread will be made of at least two plies, and it’s possible that one ply will be 100% vicuña and another 100% cashmere. However if the two types have been blended at the carding stage and spun together into a single ply, this won’t help.
Also it’s not easy to tell the difference between two animal hairs, both of which will be very soft. Vicuña is a camelid, and cashmere a goat, so there will be textural differences, but might take an expert and a microscope to tell them apart.
Sadie is absolutely correct about vicuña being protected by CITES (trade in endangered species regulations), and so not always possible to sell legally, and usually not possible to sell without paperwork, eg permits or proof of age. The exact legalities depend on where you are and where you are selling to. If you bought it in 1967, it predates CITES and so it was legally acquired, which is a good start, but doesn’t necessarily mean you are now allowed to sell it.