Extraordinary book, I've read several other books by Baxter, and this one probably covers the longest time span of any book I've ever read. A few million years here and there, and pretty soon you're talking about REAL time.
Vacuum Diagrams is a collection of science fiction short stories that collect and intertwine the author's larger works in a (relatively) cohesive whole. Baxter actually pulls this off better than many - Asimov and Heinlein come to mind as examples of those who should never have tried such a thing - probably because he actually did have a relatively cohesive world view as he wrote his various works.
Nevertheless, while I found these stories acceptable, they really didn't stick with me. There's something about huge, sweeping tales - these cover the entire history of our universe, and introduce a second, if that makes any sense out of context - that makes me want more than just short stories to get them to stick.
If you've read other things by Baxter and enjoyed them then this may be your thing. You can see the overall view of the universe(s) he imagines here. For me, though, these were simply OK stories without enough glue to make them work overall.
This book reads like a series of short stories, tenuously connected by an over-arching storyline. Why? Because that's what it is. It isn't apparent from the cover. (Or maybe it is and I'm just a doofus.)
Most of the stories are interesting. A few fall flat. The over-arching storyline falls flat, as does the ending.
Overall, it's a nice read if you look at it as a collection of sorta-related short stories rather than one big one. Too bad it's billed as the latter.