Paté and patis. That odd couple, some cilantro, cucumber, mystery meat, mystery sauce, and soy sauce stuffed in a crusty baguette make for probably the best sandwich for under P50.
Bánh mì stalls can be found on every other corner in Saigon. The one pictured has a little compartment off to one side where the lady heats your baguette over a charcoal grill before stuffing it with magic.
Locally, I think Pho Hoa sells these sandwiches, but I don’t know if they can compare.