If the vertices are in the same object, perform an "Object -> Optimize vertices...". But note, this will combine *all* colocated vertices in the object, not just the particular vertex you want. There are ways around doing *every* vertex (involving Surface -> Cut away and remerging the results), but hopefully that won't be an issue for you.
If the vertices are in different objects, you will simply have to combine the objects (Object -> Merge), as vertices "belong" to objects hierarchically in ac3d.
HTH
Dennis
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