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.