I hope she’s feeling better today. I’m not very experienced with kidney issues in pups, but I know that dehydration can cause a temporary elevation of Bun levels.
This article also says that not fasting for at least 12 hour before the test will cause elevation in Bun.
DogAware.com Health: Tests used to Diagnose Kidney Disease in Dogs
So the two factors (vomiting/dehydration & not fasting), might explain the elevated Bun.
Lots of positive thoughts that she will recover quickly.