That's a very good point. Perhaps there should have been an eighth season to truly give the war the impact it deserved (I'd rather have an excuse for another season than try to fit it all into the span we had ). As you say, though, the novels ended up doing a good job with that, as it happened.
Watching the Clock established in passing that Spoiler: DTI:WTC the Sphere Builders were the ones who gave the Borg the time-travel technology they used in First Contact. It was their second attempt (from their vantage) to prevent the rise of the Federation after the failure of their Xindi stratagem.
^ The Sphere Builders remind me a little of the Auditors of Reality from the Discworld books, who periodically pop up with a new scheme to destroy Humanity, but who aren't very imaginative - though they're very cunning in their way. Spoiler: As above, Watching the Clock It was a good move to establish a Sphere Builder connection, or so I think. It makes sense that they'd try again, and it helps smooth over the Borg's unusual strategy in First Contact when they're not supposed to be an innovative force. The Sphere Builders aren't Saturday morning cartoon villains, so they are allowed to try the same plan twice, with modifications (i.e. a species you can't make peace with, this time, one who can't be convinced that they're being used and should stop, since the Borg won't care)
Thanks on both spoilers. It always bothered me that the whole 3rd Season of ENT was pretty obviously a major climax of the Temporal Cold War arc, and yet it was never once actually described as such onscreen (even after the end of that thread threw Archer & Co. straight into the endgame of the TCW).
You should be able to plunge straight into Seekers. Seekers isn't a sequel or continuation of Vanguard, per se; there is some background information/references that comes from Vanguard, but it shouldn't be required reading.
You can plunge right into Seekers, but Vanguard introduces most of the characters from the Sagitarius and the Endeavor in Seekers. I think of Seekers as a companion series to Vanguard. IMO, reading both will give you a higher level of appreciation for the other
Star Trek: Seekers is designed to be more episodic than Vanguard was. Seekers #1 and #2 tell a two-part opening story, to (re-)introduce readers to both ships and crews. From #3 onward, the books will be more standalone (though there might be a future crossover if the series continues long enough).