It's absolutely vital for Viva to get a demonstrator built, approved and running some serious mileage before the bids for the new Northern franchise are finalised sometime in the middle of next year. As suggested above, running as a demonstrator on some potential routes would be a good way of judging passenger reaction as well as getting real service experience and convincing skeptical bidders. Failing that they need to hire a test track or heritage railway and do all the running they can.
All Northern's existing rolling stock leases probably run out at the end of the franchise in 2016 and bidders will want to offer a winning combination of quality and price, bearing in mind that the advent of the D78 will probably drive down the lease cost of Pacers and even 150s and trigger some horse-trading (and we may even see some refurb demonstrators emerging for these). Viva will want to get as many as possible, and preferably all, the bidders signed up for D78s. If they have signed with a successful bidder then they have probably placed all 75 units and their one remaining problem is to ensure they run as advertised and generate a profit.
If they fail to do so then the opportunity drops back to ones and twos for growth builds. The next biggish DMU-using franchise competition is Anglia, which is 6 months behind Northern, but the D stock could realistically only replace the smallish fleets of 153s and 156s and even there the low top speed could be a problem. West Midlands, to be awarded in early 2017, could just possibly see a major rabbit coming out of the hat with D78 replacing the 172 fleet, no doubt to howls of protest from all and sundry. After that there's only EMT, whose units I would say all need a 75mph capability, and Wales and Borders who will be expecting EMUs for Valley Lines so have no interest in any more than a short-term hire (again their other routes probably need 75mph units).
In view of this fairly poor prospect, it may be that if they don't get a deal for the Northern franchise they give up and send the rest of the D78s for scrap. Or perhaps find some overseas use.