They will: the early train leaves Sunderland just before the first Grand Central train, and the late train leaves Kings Cross just after the last Grand Central train. These are the two Grand Central trains which are "peak", attracting the highest fares.
1N96 1918 Kings Cross - Sunderland is not a peak service if you check the timetable!
As stated the 0540 departure isn't highly appealing, but it will doubtless attract some folk who
have to be in London before the first GC arrival and currently travel to Newcastle by other means.
The evening one will be interesting, but I reckon you'll find that there are plenty of passengers to go around. ORCATS wise the steady and constant rise of AP popularity may mean that this isn't such an issue, although VT will certainly increase there share of associated flows.
It'll be hard to tell if VT actually make money on this one as it's part of their franchise obligations. There will certainly be additional costs associated (fuel, shuffling dispatchers, extended traincrew diagrams).
Time will tell ... VT are lucky that the launch wasn't this week given ongoing events at Brockley Whins.