Translation:He is one of my neighbors.
Why did "She is one of my neighbors" not work here?
It could work for "È una" but uno lets you know that the neighbor is male.
Isn't "uno" a number in this case? "Lei e uno dei miei vicini" is incorrect?
Right, uno is the only word that refers to the one person you're talking about. Snookered again.
Why uno here instead of un?
Because one(1) - uno
But a/an -un/una/uno/un' depending on the gender.