The title is already the question.
How is it that I sleep well after drinking beer but I sleep terrible after drinking vodka?
I am talking about 2 - 3 German 5,5% beer and about 100 - 200 ml vodka.
Primary causes are dehydration and toxins built up in your system from the impurities in the alcohol you've been drinking, but in my personal experience and research sugar content can also contribute.
Alcohol makes people urinate more, which raises the chances of dehydration occurring. Dehydration can give the individual that sensation of thirst and lightheadedness.
Most alcoholic beverages contain small amounts of chemicals other than ethanol as a by-product of the materials used in the fermenting process (e.g., grains and wood casks). Congeners are complex organic molecules with toxic effects including acetone, acetaldehyde, fusel oil, tannins, and furfural.
Spirits have these in higher quantities than beer or wine due to the distillation process.
Depending on what specific brands you are drinking and/or what you are mixing with high sugar content can cause you to have a sugar crash contributing to the hangover feelings.
In my personal experience the mornings after indulging with top shelf spirits are notably better than those done on a budget. Bargain spirits tend to be cut with grain alcohol and balanced with corn syrup contributing to the issue, but in general higher priced spirits tend to be more highly refined (Vodka for example tends to have more filtration the more expensive it is).