Disclaimer: I have almost no experience cooking Vietnamese, this is just a distillation of various recipes I’ve only read.
Lamb steak (or other meat: just did this with rib-eye steak, was delicious)
Crushed chilli (or other form of chilli)
First cook the lamb (or other) steak until browned outside but rare inside. Set aside and slice into thin slices during the remaining cooking.
Slice onions then fry gently in oil until transparent. Add sugar (quite a lot – for two I use a couple of tablespoons at least), garlic, chilli and fry until the sugar is lightly caramelised. Throw in equal parts of vinegar, soy and fish sauce (roughly speaking, you want a little more total liquid than you had sugar), stir in and bring to the boil. Cook until slightly thickened. Add the sliced meat and stir-fry until cooked to your liking.
Serve with boiled rice and something green.