With the exception of layered drinks, most mixed drinks should stay mixed in the short time it takes to drink them. Even if a drink settles into layers, vodka will tend to float to the top. This is because vodka is less dense than most liquids in drinks.
The Concept of Specific Gravity
Every liquid has a specific gravity, or density, which is the weight of liquid per unit volume. Specific gravity can be expressed relative to the density of water. If the specific gravity of water is 1.0000, then a liquid with lower density has a specific gravity less than 1.0000. A liquid tends to float on top of liquids with a greater specific gravity.
Specific Gravity of Vodka
Vodka's specific gravity ranges from about 0.95 to 0.98, depending on the brand. This is less than the specific gravities of the liquids mixed with it. Hence, vodka will float to the top of a drink if it comes out of solution.