So is Vanguard the first ongoing TrekLit series to get a story specifically written as a finale?
If you mean series original to Trek Lit, I'd say so -- although there's been one miniseries, A Time to..., which is one book longer than the "ongoing" Vanguard.
I think The Romulan War: To Brave the Storm could be considered a finale of sorts to Enterprise, at least in the form we know it.
In the case of ATT - I think the difference from VAN lies in that ATT was a set of 4-5 different stories by 6 authors, where each could have worked as a separate novel / duology. Not so, with VAN, which had told a tight inter-connected multi-arc but still *single* story by one set of authors basically working as a team.
In the case of ENT- where VAN was brought to its conclusion in practically the way its creators planned (going by the interviews and podcasts), ENT's "concluding" story (if it had indeed been planned to be that) was shortened and changed by Editorial.
That being said, and in light of the VAN authors interviews, I would love to get Michael Martin's input on the ENT Relaunch as a whole, including some details on the departure of Andy Mangels.