According to Nan Flanagan vampires can live off True Blood alone; “Now that the Japanese has perfected synthetic blood which satisfies all of our nutritional needs there is no reason for anyone to fear us. I can assure you every member of our community is now drinking synthetic blood." Liam has a very different POW. So who is correct? Judging from Eric needing to feed of Sookie to heal in season 4, I guess Liam is closer to the truth than Nan.
I don’t think Nan was being on the level when she made her claim. She made it on TV, in her role as the public face of vampires, in order to appease the human population and put their fears to rest.
As we have seen in reality, however, Nan herself was hardly living off Tru Blood.
Having said that, the bombing of Tru Blood factories would not have been such a devastating disaster, had this substitute not been effective at least partially. Clearly plenty of vampires were using it regularly.
So, perhaps, the truth is in-between somewhere. They could live off it, but not forever, and not without some supplement of actual blood. Kind of like Ensure.
I think Tru Blood does fill the very basic nutritional necessities for vampires — it can keep them alive and keep their hunger in check. However, Tru Blood doesn’t contain any living cells so it certainly doesn’t help them to heal. And, I’m betting, other vampire super powers begin to wane if one lives off of just Tru Blood — their strength and speed probably diminish to nearly human levels. They might lose the ability to glamour, levitate, fly, whatever else powers they have (I can’t totally keep track :P). Alternatively, however, I think feeding on live blood makes them more violent and inclined to kill.
So I think Tru Blood is sort of like an unpleasant diet and bombing the Tru Blood factories was a disaster (from a human POV) because forcing the vampires living just on Tru Blood to feed on humans again made them both more powerful and less docile.
Like Eric said: “It keeps you alive, but it will bore you to death.”