Sweet vinegar: Sweet vinegar or "amazu" 甘酢 can be made ahead. It keeps a long time in the refrigerator. I put rice vinegar in a non-reactive (such as stainless steel or Pyrex) pan on low flame and added sugar (half the amount of vinegar, either by volume or weight, for example, one cup of vinegar and 1/2 of sugar) and a small amount (I used 1/3 tsp but could be more) salt. Stir and make sure the sugar is completely dissolved and let it come to a boil (called "nikiru" 煮きる), this makes the vinegar mellow. Let it cool down and put it in a plastic or glass container and keep it in the refrigerator. This can be used for many other recipes.
Marinade: I mixed sweet vinegar (2/3 cup), dashi (1/4 cup), mirin and soy sauce (1 tbs each). I added thinly sliced red onion and julienne carrot the night before. You can do this part a few days ahead. I like the veggies to marinated at least several hours or longer. To make it truly "nanban" you add red pepper flakes but I did not this time.
While the fried shishamo is still hot, I put it in the marinade with the vegetables already in. I cover the fish with marinated vegetables and let it marinate for at least 10 minutes or longer. I served it with the vinegared onion and carrot on the top.
We had this with cold sake (our house sake Yaegaki "mu"). I think sake or beer will go with this dish well but the acidity of the dish does not agree with wine. Of course every part of the fish including head, bone and tail is eaten in this dish. "Waste not want not" never tasted so good.