I must congratulate you for having succeed in training her to vomit in her litter box. Even if it is about 50% of the time, it's a great achievement.
I have one out of three who not only is susceptible to hair balls but is also very ocd-ic about it. All three groom a lot. They are very proud of their beauty. π. I also brush them regularly. I clean the floors, couches, their cat trees, their beds, blankets, everything. Some every day, the others every second day. And there is always some new hair there at the end of the day. While two of them don't even notice it, the third one constantly and obsessively turns into a feline vacuum cleaner and eats all the hair she can find. Hers and theirs. I litteraly have to pick her up and move her somewhere else and rwmuve the hair. She is the one who has problems with vomiting hair balls.
I give her malt paste, she doesn't like it. I give her dry and wet hair that is supposed to help with the issue. I even have ocd therapy with her. I talk to her. I give her a lot of affection and hope she feels secure and confident enough to stop doing that. I don't know what else to do.
Apart from that, she has another ocd like issue. When she pees in her litter box, she just jumps out and goes away without covering it with the sand,... But when she does number 2, she never covers it with sand either, but she compulsory scratches on everything around her( furniture, box,...) as if she was actually covering the poop with sand. She can do that for 5 minutes at a time. She never touches the poop or sand around her.
Funny is that the other two, who do it the way cats are supposed to do it, first cover their poop the regular way, and then just to be like her, scratch around on the floor, their furniture,... It's funny to watch.
I do make sure they have fresh cat grass and fresh water. After eating the grass, all three sometimes vomit some hair, but the vets say that is actually good.
So dear 777Q, we are in the similar boat. I have no idea what else to do. If you come up with a solution, I sure would appreciate it.
It's never boring when one has cats.