Hello, I have recently changed my budgies food from viral to a different brand, ever since I’ve had her she’s been fine, a shy bird  but I figured she’s getting used to us and my other female budgie, who we have had longer therefore getting them to become friends was a long process in its self. 
recently as I changed the food my blue budgie started vomiting, it’s only happened once today and it’s the first time I had seen her vomit as I have seen them both regurgitate and I knew it definitely wasn’t regurgitation, she was flogging seeds and head bobbing. I was just wondering could the change in food be a cause? She is younger than my yellow budgie, or could this be a more serious thing? 

Yes actually. It is advisable that you change back to your former brand

Happened the same with my budgie, she even started sneezing after vomiting. I changed her diet to the former one and she was fine in a couple days.