I'm not sure if the pricing model is the same as it used to be but when T3 were operating NCL it was said that the breakeven load factor wasn't that high, even with only 29 seats. The starting prices currently available is £79.99 one way, and according to the website it looks like only 4 seats are available at that price. Hopefully selling 29 seats isn't going to be that difficult as the flight timings aren't too bad, despite it being only once a day. NCL worked for T3 for many years despite LF's not being that high, and i would have thought there is more demand for Belfast than Newcastle.
Depending on crew availability there is scope for another evening flight to BHD or elsewhere, as the VLY aircraft arrives back at 1820, so a 1900 departure to say BHD or EDI would work. That would then utilise 2 sets of crew throughout the day, which I'm guessing 2 sets of crew is already required for the current offering of VLY x2 and BHD x1.