So sorry to hear about your poor lamb. You obviously cared a lot for it.
As to the vet, well, I agree with the others. I know I can ring my vet in an emergency, and he will be out within the hour without doubt. If he is out on another call, he will ring my mobile and give me advice, ask for an update on the animals condition etc. Your lamb was an emergency case, and your vet should have reacted accordingly.
I think you are upset, and feeling bad about the loss, so want to know if it could have been saved. I think the honest answer is probably no. But the vet would have put it to sleep for you, stopping any further pain and suffering for both the lamb, and also you. It was obviously distressing for you.
I can understand you hand rearing the lamb, but having it in the house (although I have had them in a box by the rayburn as well as goat kids, in the kitchen) but once reared they go outside in the field. I have a pet sheep now, but he is out with the llama and goats, although he is still tame.
Your lamb would not understand what is poisonous and what is not, and it seems you were not aware that what he ate was indeed bad for him. I do hope that before getting any lambs you read up on what they can and can't eat, and how they should be kept. I am not sure if you are in the UK, but if you have been taking the lamb to schools etc, you should have been keeping a movement book updated each time he went away from your house. And was he ear tagged, and did DEFRA know you had him ......what about a CPH - holding number? All this is irrelevant now, but things to make sure you have in place for any future lambs.
Sorry if this sounds like a lecture, you will already be feeling bad about your loss, and I am truly sorry for that, but what I have said above, are required by law if you keep livestock.