What we know:
1. Vulcan has been destroyed, and at least seven Federation starships were lost defending it.
2. Forty seven Klingon warships (types unknown) were destroyed.
3. When Vulcan was attacked, Starfleet was heavily deployed in the Laurentian system. It can be implied that they were conducting combat operations there, but this would involve a fairly literal reading of the words "engaged in."
What we don't know:
1. How many Vulcans lived off of Vulcan itself? It could easily be millions upon millions, scattered across colonies and other Federation member worlds as ex-pats. Of this unknown number, how many will return to nuVulcan?
2. How much industrial/economic/scientific weight did the planet Vulcan itself hold within the UFP in 2258? How easily can the Federation absorb the loss of the planet? Related to this, how much military manpower will be needed by the Vulcan diaspora to settle a new home?
3. Given the catastrophe at Vulcan, can the Federation somehow extract itself from the Laurentian situation and deploy the fleet closer to home? They no doubt wish to, but circumstances may not permit it.
4. How serious a loss to overall Klingon strength does 47 starships constitute?
5. How long was the Narada orbiting Earth for, and while there, did she engage any starships or space stations? We know there was at least one giant station in orbit when the fleet left, is it still there?
Reasonable conjecture (in my opinion):
1. The seven starships known to have been lost at Vulcan probably do NOT constitute the extent of Starfleet's losses there. No doubt Starfleet had facilities and stations in the Vulcan system, they have probably been destroyed. Indeed, the massive saucer section that the Enterprise had to maneuver under when it dropped out of warp was so massive as to have almost certainly been part of a space station or starbase, perhaps one of a similar design to the one seen over Earth. Also, given the amount of debris in orbit, it's certainly possible that Vulcan also had a half-dozen or so starships handy when the Narada arrived, which were lost. (Or, perhaps the eight ships sent from Earth were just part of a larger rescue operation, with any available ships from other nearby UFP systems arriving before the ships from Earth did, and facing immediate destruction).
2. Starfleet will likely be recalling ships from exploratory missions to bolster local defences.
3. The Romulans are probably finding themselves in a bit of a pickle. The other two major powers have taken heavy blows, and they time may be right to attack.
4. Likewise other minor powers throughout the Klingon and Federation frontiers.