Things Man Was Not Meant to Do is a falsehood born of fear towards what we do not yet understand. It is fully within our rights to utilize magic, whatever form it may take, for our purposes. It is not divine retribution which we should fear, but that of our own kind who may or may not consider our cause just. User, I won't lie and tell you this is a simple issue. You clearly intent good and wish to restore what has been lose by those you love. I am very sorry to tell you everything that I must.
I do not know if a body still has a soul while it is in a coma, though I would personally guess that a vegetative state from which someone cannot wake up is more likely to have passed on. Even if this body is vacant, there is another reason why you cannot call the other soul back. It is because he does not want to return. The afterlife is a wonderous place that I cannot describe in any reasonable terms, you must discover the knowledge of it yourself. I have learnd from astral projection and Sylvia Brown.
The soul you are calling is near, but not because he wants to return. He is most likely watching over his family or maybe trying to communicate with you. Hauntings are rare because the dead are so often content with their lives and aware of their existence and purpose beyond single lives that they are...well...happy to be dead. Even in the very rare cases where they aren't, their obsession with their former lives is extremely unhealthy and frequently drawn to the afterlife by their wish to be free of what they couldn't let go of.
I am so very sorry that your spell has failed, but there is one final course of action you can take. Talk to him. Try to make contact with the soul you're calling and ask him what he thinks and wants. While you're at it you might as well speak to the soul of the comatose body. Talking with the dead is not easy for those who've never done it before, but I have managed to remain in contact with my deceased father through astral projection. Once again, I am so sorry that you hear this in as painful a way as you must have.