"Marcus is half-asleep or stupid."
Translation:Marcus est semisomnus aut stultus.
Good, that's what I thought. Now I wonder what the semantic difference is, in (either) English or Latin? It seems to me that the 'either' mainly adds a sign to the speaker that the following is a multiple choice between options. Leaving it out doesn't seem to change the meaning in this context, but just makes it more of a road-bump to parse.
Marcus, I agree that 'either' is more restrictive, but I'm not sure it's quite in that way. If we say "Marcus is half-asleep", we don't imply the possibility that he may be stupid instead, and if we say "Marcus is half-asleep or stupid", then we likewise don't allow for the possibility that he is actually a very sharp and wide-awake fellow who is just yanking our chain. We can allow for that by adding another 'or', but we could do that as well when we use 'either'.
I wonder if 'either' doesn't imply mutual exclusivity in a way that plain 'or' leaves fuzzy? "Marcus is half-asleep or stupid" leaves open the possibility that he is both, but "Marcus is either half-asleep or stupid" makes it a radio-button where only one listed option can be chosen.
Vel works, too, as a disjunctive that introduces an alternative. But aut is fine, one is not necessarily better than the other. Your reasoning about vel matches the example Bennett gives: quī aethēr vel caelum nōminātur. Have a lingot. http://www.thelatinlibrary.com/bennett.html#sect341
I appreciate DL Latin using semisomnus. The Romans liked to use "semi" in compounds and that is one of them. They also liked to insult each other with semi compounds, as Jerome does in Ep. 41.4 referring to Montanus as semivirum. Another Hieronymian insult, Dormitianus, often rendered "sleepyhead," is an obvious play on the name of his enemy Vigilantius. Sensitive to Latinitas, he sometimes referred to Latin mixed with a local vernacular as semisermo.