Not P but Moby-Dick (44)

[Mike Jing]

>From Chapter 54:

“At sunrise he summoned all hands; and separating those who had rebelled
from those who had taken no part in the mutiny, he told the former that he
had a good mind to flog them all round—thought, upon the whole, he would do
so—he ought to—justice demanded it; but for the present, considering their
timely surrender, he would let them go with a reprimand, which he
accordingly administered in the vernacular.

What does "in the vernacular" mean here exactly?