'Vreunul' counts for the idea of somebody or something taken from a group like in the following context. Say, I've brought you some candies and I am curious if you enjoyed one type in particular: Ti-a plăcut vreuna / una (=one) mai mult (in particular/more than the rest)?
1. We use 'cineva' for persons and 'ceva' for things, objects, actions.
2 'Vreunul' is a compound < vreo + unul/una (= a type of one / something or someone in particular).
We also have this saying/idiom:
Dacă-ți spun vreo două (= 2 words), meaning that I am upset and I warn you that if you go on I will say some heavy words to you so, don't provoke me.