Your homemade vinaigrette is likely too thick due to an over-emulsification of the oil and vinegar. To thin it, gradually whisk in small amounts of water or additional vinegar until you reach the desired consistency.
A vinaigrette is a temporary emulsion of oil and vinegar, meaning it naturally wants to separate. When you whisk vigorously or use a blender, you break the oil into tiny droplets that are suspended in the vinegar. Sometimes, this process can go too far, creating an emulsion that's too stable and thick. This can also happen if you use ingredients that naturally thicken the vinaigrette, such as honey, mustard, or certain herbs.
Here's how to thin it out:
To prevent your vinaigrette from becoming too thick in the first place, whisk it gently by hand instead of using a blender or food processor. This will create a looser emulsion that's less likely to become overly stable. Also, add thickening agents like honey or mustard sparingly.