I don't see rebuilding as much of an issue. The world has been in ruins many times ITRW already, and if anything, WWII was a case of the easiest rebuild of all history - we had surplus technology galore, and cities were up and running basically overnight, it taking just a couple of months to re-establish basic infrastructure (water, electricity, plumbing) in many places, and a few years to erect the buildings again.
600 million casualties combined with all the big cities being in ruins is a perfect combination IMHO: the qualifying limit for "big city" could very well be 10 million inhabitants, the world could well have 60 of those at the time, semi-rural places like London or New York would not qualify, and the damage would essentially be limited to said big cities. Perhaps the reason there is no China in Star Trek is because the sixty cities were all theirs?
No need for Vulcan help in rebuilding. And no evidence that they would have helped in any major way. From ENT, it seems they run a game of client states, maintaining a sphere of influence without overtly taking charge or directly providing support. But there's no real evidence that they would have held back, either - that's just young Nat Archer's misreading of the situation, something his dad would cringe at whenever the annoying kid brought it up. There could have been plenty of small-scale assistance, cures for cancer, antigravity belts in pretty Slaver boxes, good advice on how to proceed logically. But common folks would just see the lack of unobtainium ore shipments in the sky or pointy-eared laborers at the construction sites.
If we didn't become part of the Vulcan sphere of influence, we probably would of somebody else's. Vulcans in ST:FC seemed to claim (or Troi suggested they did) that Earth was uninteresting until it discovered warp; "Carbon Creek" reinforces the impression. It probably became a nuisance at that point, so Vulcans took charge. But Andorians would take charge if Earth helped in the fight against Vulcan. In both cases, the sphere of influence would be a protective shell against general random invasion, even if a magnet for Andorian or Vulcan aggression, respectively. And DSC would suggest Vulcans at the time were quite capable of keeping, say, the Klingons at bay, while ENT shows the Andorians as their peers.
How could things go differently without the Vulcans? That is, how could Vulcans let something else happen to Earth? We might be a pawn in the game against Andoria, handed over for gain, and we'd again be fine. But if some Klingons barged in here first and declared our planet theirs, would either Vulcan or Andor bother? I guess they would: their antics in "Cease Fire" suggest they could not afford not to.
I also guess the interesting question here is, what kept Earth safe in the years before the warp discovery? Back when we weren't even a nuisance or a pawn to the Vulcans? Were we really too uninteresting to the Klingons, even though they live almost next door? To the Romulans, who were at Alpha Centauri in the alternate timeline of "Past Tense"? To any of the random folks introduced in ENT, including those fleeing from the Expanse which was just 50 ly away or so? Whatever our guardian angel's true nature, it probably would significantly affect our fate in case Cochrane failed to give the Vulcans the spread fingers.
Timo Saloniemi