In a small pot over high heat, combine the vinegar, water, salt, and sugar. Stir to combine and boil until the sugar is dissolved, approximately 3-5 minutes. Remove from heat and set aside.
Meanwhile, cut the carrot and radish into matchsticks. Make sure they will fit in the mason jar or container you are using. I made mine approximately 4" long and 1/8" thick.
Mix the carrots and radish together, and put them in two pint sized mason jars.
Slice the serranos thinly and place one pepper worth in each jar with the carrot and daikon.
Pour the vinegar mix over the vegetables. Cool to room temperature, screw on the lid, and refrigerate.
Refrigerate at least one hour before serving, or up to five days. The longer you leave them, the softer the vegetables will become and the spicier they will be. Adjust the amount of serrano that you use based on how long they will be sitting and how spicy you want them.