Before anything goes into the blender, I like to rinse my basil leaves and pat them completely dry. Wet basil waters down the dressing and dulls the flavor. I also peel my garlic and roughly chop the shallot so the blender doesn't have to work too hard to break everything down evenly.
Add the basil, shallot, garlic, red pepper flakes, olive oil, red wine vinegar, and salt straight into your blender. Blend on high for about 60 seconds, stopping once or twice to scrape down the sides if needed. You're looking for a smooth, pourable, vivid green sauce with no visible chunks left behind.
Once it's blended, I always dip a spoon in and taste it before calling it done. If it needs more brightness, I add a splash more vinegar. If it tastes flat, a pinch more salt usually fixes it. This step matters more than people think, since basil varies in how peppery or mild it is.
You can use the vinaigrette immediately while it's fresh and vibrant, or transfer it to an airtight jar and pop it in the fridge to firm up slightly. I usually let mine chill for at least 15 minutes if I'm using it on a salad, just so the flavors settle in.