Combine white vinegar and milk alternative and set aside (this functions as buttermilk).
Coat a large pot with cooking spray and set over medium heat. When the pot is hot, add the oil, onion, and then the garlic. Cook, stirring occasionally, until the onion is soft but not brown; about 5-6 minutes. (Don't burn the garlic!)
Add broth and potatoes and bring to a boil. Reduce heat and simmer, partially covered, until the potatoes are tender; about 25 minutes.
Carefully spoon the mixture into a food processor or blender and add the vegetable purees, the milk mixture, and salt. Puree until smooth.
Ladle into bowls and sprinkle with croutons, if you like. Serve hot.
Notes
Directions for making purees - *Butternut Squash: cut off the stem, cut squash in half lengthwise and scrape out seeds. Roast halves on cookie sheet, flesh-side down, at 400 for 45-50 minutes. Scoop out the flesh and puree in food processor or blender for about 2 minutes. *Cauliflower Puree: Cut off florets and discard core. Steam for 8-10 minutes. In a food processor/blender, puree for about 2 minutes with a few teaspoons of water if you need it for a smooth, creamy texture. •NOTE: I often make large batches of purees and package them in Ziploc quart freezer bags in 1/4 or 1/2 cup size amounts. Then, you can simply thaw them out for a recipe and snip off a corner of the bag and squeeze the puree out of the hole into a bowl. Very easy and efficient! (Recipe adapted from "Deceptively Delicious" cookbook by Jessica Seinfeld.)