Plus ultimately if you are getting hung up on the age of David Marcus and that is preventing you from enjoying either Inception or the Vanguard novels, then you need to step back and re-evaluate your approach to this stuff IMALTHO.
Who said anything about enjoyment? I'm only talking about continuity. It is entirely possible to enjoy two mutually contradictory works equally. I enjoy
Batman: The Animated Series and I enjoy
The Dark Knight, even though they're in incompatible realities. I enjoy the '90s FOX-Network
Spider-Man animated series and the more recent
The Spectacular Spider-Man animated series even though they're radically different takes on the character and continuity. I enjoy John Byrne's Romulan comics for IDW and I enjoy the
Vanguard novels, even though they present incompatible versions of Romulan/Klingon history and politics.
Someone asked what kept
Inception from fitting into the main novel continuity. I answered that question. That's all. Purely a matter of continuity, no criticism intended. There are many different continuities currently or formerly in play in Trek literature. There is the current main novel continuity. There is the Shatnerverse. There is the
Crucible trilogy. There is the Mirror Universe and the various
Myriad Universes tales (although the current iteration of the novels' Mirror Universe is tied in with the main novel continuity). There is the Abramsverse as represented by the
Starfleet Academy young-adult novels. There is
Star Trek: Online as represented by
The Needs of the Many. Over in the comics, there is IDW's John Byrne continuity. There were several notable continuities in the past that are no longer compatible with canon, such as the DC Comics continuity and the '80s novel continuity which was anchored by
The Final Reflection, the Rihannsu novels, and others. And there have always been assorted standalone tales unconnected to any particular continuity.
So saying that two works occupy different continuities isn't meant to say that one is better than the other or that they can't both be enjoyed. It's merely a matter of categorization, keeping track of what goes where.