It sounds like this is regurgitation rather than vomiting
A bird that bobs its head up and down in a sort of pumping motion, beak open and then a purposeful delivery of partially digested food is regurgitating. This is something they would do to feed offspring or a mate. It’s done by some birds to objects they are particularly fond of, especially if they’re in a breeding season. It might also be done when the bird is nervous or trying to ‘please’ you.
Regurgitating can also be a symptom of crop infection or other problems, but generally it’s a threat because it can lead to malnourishment. If your bird is giving up too much of the food they should be digesting for their own nutrition, the result can be pretty serious.
Vomiting is more of a head ‘flicking’ event. The bird will often seem uneasy, pacing or uncomfortable and although the head bobbing might be similar to the regurgitation action, it’s usually more of a shaking and the end result is a very splattered, sticky substance that may or may not include food.
If possible, collect a sample of this substance to bring to the vet with you.
When there’s a blood showing in the vomitus it may indicate esophageal or proventricular ulcers (http://www.multiscope.com/hotspot/pdd.htm) .
If you’re dealing with regurgitation, as I believe you are, try to identify what triggers your bird to do this. It may be a shiny object, mirror or something he does when he sees you approach. To help make this stop, once you figure out the cause and remove this object of your bird’s affection - you can also begin to modify the bird’s light and dark hours to help curb this behavior.
By changing the cage around, switching up the food and water dishes, taking out familiar toys and adding new ones, you alter the possible triggers. Change the location of perches, or the types, but always keep in mind that the highest perch where the bird will spend most of their standing time should never be a coarse or grooming perch which can cause foot and leg problems. Those perches are excellent options in a cage, just not for long time standing or sleeping.
If it's you or someone else who seems to trigger it, when he begins to bob his head, stop interacting with him, become very quiet and turn your back, or even walk away. By paying less attention to it, you help discourage it.
Take a look here for more about sleep, sleep cages, open perches and lots more www.4AnimalCare.org/birds
Hopefully this will resolve itself without having to go through more injections or medications, but I commend you for being responsible enough to have sought out a vet to begin with. It proves you care a great deal for your companion. Nice job!