From the man himself on twitter. Spoiler An all-new Star Trek novel—continuing the legacy of the critically acclaimed Vanguard series! Captain James T. Kirk and the crew of the Starship Enterprise go in search of a missing Federation scientist—only to become trapped between a Klingon warship and Starfleet’s mysterious Operation: Vanguard. Some spoiler content in the book description, in that it references something I thought was done being referenced...
I'm so excited about this David Mack book news this is the type of Tos Spoiler: Tos spoilers I wanted to see with continuing the Vanguard series returning again along with having a mystrerious Scientist and Kirk and crew being involved after the Doomsday machine will be fantastic[ to see the Vanguard characters again in a new story/SPOILER]story telling I've wanted to see continue in the Tos books.
I don't mind leaving the thread open as the pre-release discussion thread for the book, if you'd like. If you prefer it closed, though, I can oblige.
Spoiler: Coda If you're referring to the whole Picard/Coda thing, the timeline that got erased only started in 2373, so Vanguard still happened in the prime timeline (according to the novels). Although it will probably be contradicted soon by Strange New Worlds, but not yet!
The First Splinter began in 2373. This novel is set more than a century before then. If Strange New Worlds does end up contradicting Vanguard, we'll have to find some other explanation. Or just, y'know, accept that it's all just stories and they contradict each other a lot.
Although the first splinter officially begins in 2373, it's existence depends on events in Destiny which cover a much larger span. Oblivion's Gate suggested the First Contact Borg may have had a different origin to the Destiny Borg, so I think it's a valid suggestion that it goes much further back.