Although multicast MAC addresses don't usually belong to any vendor, they may be used by the companies who own their unicast neighbors. Typically, such addresses are used for various reserved purposes, within custom protocols or equipment. Change the least significant bit of the most significant octet to 0 in order to get the corresponding unicast OUI - and you will know the vendor. Note that the mechanics described might not work for all the multicast OUIs.
Here is a table of some multicast-unicast OUI twins we discovered:
and many more...
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