What is the reason the pulmonologist added budesonide after levalbuterol/saline, vest, airway clearance?

Budesonide is a steroid. The reason to use a steroid is usually because you have asthma in addition to everything else.

Do you have MAC? It is my understanding that those with MAC should not use steroids.

Regarding thrush, with steroids you do need to be careful. Rinse your mouth well immediately after nebbing the budesonide. Getting the rinse mentioned above is a good idea as is eating yogurt a couple times a day, letting it sit in your mouth before swallowing.