I didn't think that oncidiums could be crosses into a vanda .The oncidium is in the cymbidieae tribe and the vanda is in the vandeae tribe i would think it would have to at least be in the same tribe to cross and even then it doesn't always work.. I think i would have to see it in real life before i'd buy it or believe it.
Hate to say it, but the seller is not great when it comes to correct names on his Ebay sales. I have seen a Dendrobium Anucha Flare listed as Cattleya by this seller on Ebay