I would agree that the sandwich needed something more and was a little dry. I've never tried the mayo mentioned, but it does sound like just the thing.
Prepped to recipe except hollowed-out the baguettes and crisped them in 450° oven while the steak rested. Added the meat juices to the vinegar along with steak. In assembling the banh mi, poured some of the vinegar broth over the beef, then topped with the vegetables (incl a few cuke slices). Tasty, but dry. Leftovers made much better sandwiches the next day with a bit of the mayo/chile spread from CL's Sep '08 banh mi.