Making salad dressings, sauces, mayo are similar. As the ingredients mix the oil combines as the acidic ingredients combine the sauce thickens. tips to make it thick, pour the oil slow, or use an immersion blender. have patients, try adding a little more oil. Getting the hang of it can take a couple of tries, If you’re making small batch items, a vitamix may be too large and not have enough depth of ingredients to mix properly. Try an immersion blender, small blender like a bullit drink blender. Without more info it’s hard to say what was the problem. Try a different oil, I use a light olive oil, avocado oil or vegetable oil.