I transplanted a juniper from a nursery container to a bonsai dish in a class, yesterday. Should I water it with a solution of vitamin B to treat it for possible shock? I didn't think to ask at the class.

Depends on what you believe.

"Vitamin B1 (thiamine) is one product that makes claims that cannot be supported by science"

cals.arizona.edu/yavapai/anr/hort/byg/ar...stimulators2015.html

"Applying vitamin B-1, or thiamine, to root systems of whole plants does not stimulate root growth. This
is a myth that refuses to die, though it has been repeatedly refuted in the scientific literature"

s3.wp.wsu.edu/uploads/sites/403/2015/03/vitamin-b1.pdf

Is there something I should feed the newly potted (actually a bonsai dish) tree to help it along?

The simple answer is no.