Nwoody,
Firstly, Vueling have 2 A320 configs. One has 180 seats, the other has 186 seats so worth checking what operates each flight. Also the A321 is 220 seats IIRC.
I've not seen any evidence to suggest Vueling were looking at a BHX base, and I don't think we can make any assumptions based on the difference of 1 weekly flight. It also looks like BCN could be going 9 weekly from MAN, as their online timetable shows a late Friday flight as well as the afternoon flight once more.