I flew NCE-BCN on October 26. I booked it on October 6 and the basic fare was €23 so I had the bag restriction but it seemed like if I pre-booked my seat I would be able to bring a carry-on and a bag to go in the overhead bin, so I did that on October 7 - I booked seat 4A for €18. In the intervening 2 weeks Vueling sent me many contradictory emails advising me I would only be able to bring one cabin bag / would I like to pre-book my seat / would I like to pay for extra cabin baggage as it was going to be weighed etc.
My issued ticket showed group 1 boarding but no mention of being able to bring 2 bags into the cabin. On departure day I went straight to the gate and about half the passengers seemed to be group 1, many (including me) with 2 cabin bags. The check-in staff just let all the group 1 people on with no baggage counting or weighing, I don't know if they restricted the other boarding groups. So it seems like the best option is to pre-book your seat. YMMV but I was able to fly up-front with 2 cabin bags for €41.