The night before you want to serve, begin your prep by bringing a large pot of salted water to boil. Cook the elbow pasta according to package instructions. If the package gives you a range of time to cook, use the longer cook time for a slightly softer pasta.
While the pasta is boiling, put rice vinegar, sugar, and dashi powder in a small saucepan. Heat and stir on the stovetop until the sugar is dissolved.
When pasta is done, drain and put in a large mixing bowl. Add rice vinegar solution and stir to mix. Cover with plastic wrap and let the pasta soak overnight.
The next morning, add in your chopped veggies and imitation crab to the pasta along with the entire jar of mayonnaise and tablespoon of sesame sauce, if using. Stir, cover, and put back in the fridge to chill until serving. Leftovers keep in an airtight container in the fridge for a few days.