Time. And your hands not doing anything he sees as threatening. I’m not at all familiar with Vasas but I know that birds have very long memories and they perceive things as threats that would never occur to us.
He gets himself wedged behind the desk, you have to get him out. We see that as “I saved you” he may see it as “she tried to eat me!” The vet trips may play into it as well.
I raise a lot of prey animals. Rabbits, horses, chickens and it’s the same with all of them. I could have bought a nice car for what I just spent saving my stallion. His thank you was to put my daughter the hospital for trying to pick up his foot to clean it. He had a lot of painful procedures over an 11 month time frame instead of understanding that they literally saved his life, he simply equates people touching his legs with pain. It will take a year or more before it’s safe to handle his legs.
I know time is a stupid answer but I believe it’s the correct one.
The other thing I will note is that all of my beaky birds bite harder the longer they hold anything whether it’s a toy or me. I have one bird, Skyy, who well understands that biting is painful. I know she knows because she will tell the lovebird to “stop that! It hurts!” With her, ignoring biting didn’t work. But if I say OW! She will release and move away.
It could be that Java wants the hand to leave, it could be that he wants to play with it or he wants a treat or just a reaction. When Skyy got beaky, I left her alone. Eventually (after months) she came to me. You’ll develop your own language with him after some years and come to all sorts of understandings. It’s just all about time.