I love keeping a variety of flavor condiments around like this version of Thai chili vinegar. You can add it to stir fries, soups, sauces, noodle dishes (this is GREAT for drunken noodles), and more. Some recipes don't add water, but since I tend to use this as a base rather than a stand-alone sauce, I like to add a little extra liquid.
The best peppers to use for this are red or green Thai chilis, also known as Bird's Eye chilis. They are small and pack a satisfying punch that hits you quickly but wont linger on your pallet as long as, say, a habanero pepper. If you can't find Thai chilis, you can also use Serrano peppers or even small green Jalapeños.
The full recipe can be found under the video.
~1 cup Thai chilis, thinly sliced
~1-2 cups white vinegar
~1-2 cups water (you can add much less water if you like, but as stated above, I like to add this to create more of a chili vinegar base rather than a sauce)
A couple tbsps white sugar
A couple tbsps salt
2 cloves garlic, thinly sliced
Boil water and combine with your vinegar, salt, and sugar, and give is a quick stir. The salt and sugar should dissolve quickly in the hot liquid. Add your sliced garlic and chilis and give it another stir. I like to include the chili seeds for extra spice, but you don't have to.
Store in the fridge for up to a few months, shaking and "burping" the jar (opening and closing it to release any air) every few days.
This recipe is very similar to that of a quick pickle (i.e. vinegar, salt, sugar, spices). The salt and vinegar will help preserve the garlic and chilis longer, and you can add more chilis and garlic to the brine overtime if you'd like, as well as other spices.