Vodka sauce can be thickened with flour by creating a roux. To make a roux, melt equal parts butter and flour in a pan on about medium heat. Stir the combination until it starts to bubble. Then, let it cook for 2-3 minutes, stirring occasionally, until it turns light brown. Once the preferred color is reached, slowly add the vodka sauce while whisking continuously to ensure the ingredients are evenly combined. If the sauce is too viscous for your preference, thin it out with additional liquid such as broth or water. Be sure to add only a little liquid at once, or you may complete it with a runny sauce. Additionally, if you're looking to reduce fat content, you can supersede half of the butter with olive oil or avocado oil; however, this might result in a slightly different flavor profile than using all butter. Lastly, maintain an eye on the temperature of your sauce and do not let it boil, as this can cause lumps to form.