What was in it, I can't say for sure; it was fishy, perhaps even fermented, and just a tad spicy. Compared with your typical New York banh mi, this sandwich was slathered with much more sauce and mayo; the veggies were chunkier and laced with onion, too.
Banh Mi Sau Minh
Vo Van Tan, Ho Chi Minh City
(From a November 2006 visit)