It may well be Nuoc Mam. This Wikipedia page
Nước chấm - Wikipedia, the free encyclopedia describes it, and it sounds pretty similar. After reading the Wikipedia page, I remembered that it definitely has some garlic in it, too, and it is served with the springrolls, which seem pretty Vietnamese. Wow, I spent a fairly long time on the internet without finding the answer. I will try to recreate it at home, I hope it turns out delicious.