Hola David

As the lesson is saying on one note: the possessive agrees with the noun it is modifying, so in this sentence:

I am a neighbour of yours (when yours is referring to you plural) = Yo soy un vecino vuestro

That "yours plural" could be referring to you plural feminine or you plural masculine, but whatever the case, it will still be "vuestro" because it agrees with "vecino".

Now, you could also say:

"Yo soy vecina vuestra" (I am a neighbour of yours) 

But in this case you are saying "I am a "female" neighbour of yours), in which case you then need "vuestra" (not vuestro) to agree with feminine noun "vecina".

I hope this helps,

Inma