In a large skillet set over medium heat, toast the árbol chiles, stirring them slowly and regularly until they release their aroma and darken in spots, usually about 1 minute. Scoop into a bowl. Toast the guajillo chiles, pressing them flat against the hot surface until they lighten in color on the inside and release their aroma; flip and toast the other side. Scoop in with the árbol chiles, cover with hot tap water and weight with a plate to keep them submerged. In the same skillet (still over medium heat), roast the garlic, turning the cloves from time to time, until they have softened and are blotchy-black in spots, about 15 minutes; cool until handleable, then peel off their paper skins. Scoop the garlic into a blender jar. Use tongs or a slotted spoon to transfer the chiles to the blender, then add 1 cup of their soaking liquid. Blend to a coarse puree—nearly smooth, but with a little texture left. Scrape into a serving dish and stir in the vinegar. Taste and season highly with salt, usually a generous 1/2 teaspoon.
Morita salsa: You can make a really delicious, sweet-smoky version of this salsa by replacing the árbol chiles with ½ ounce (4 medium) dried morita/chipotle colorado chiles. Treat them as you would árbols, though they will require a little longer to soften in the hot water than the 15 minutes it takes to roast the garlic. I like to add a pinch of sugar to this bold salsa to bring out the natural fruitiness of the chiles.