wow, this thread went nowhere logical.
1. Being dehydrated simply means you don't have enough water in your body, that is all, plain and simple.
2. Your urine is the prime indicator of just how dehydrated you are. The darker it is, the more dehydrated you are and the more fluids you need to drink. You are not properly hydrated unless it is clear or almost clear. Please note that some foods and medicines can alter urine color though. Urine is the first indicator of being dehydrated, a dry mouth, headaches, and extreme thirst are later symptoms. If your getting headaches without the darkened urine, then they likely aren't being caused by dehydration. A hangover is caused by dehydration if you need an example of it causing headaches.
3. Saying you *have* to go to the hospital if you are dehydrated is like saying you have to go to the hospital if you have a fever. It's not true, a fever is fine unless it's nearing 108, over that is when bad things can happen, like brain damage, coma, and even death. It's the same with dehydration, you don't have to go to the hospital until you get to the point of passing out. Before that, it can be fixed by drinking water. Heat Exhaustion in the summer is a prime example of this. You get dehydrated to the point your sweat output decreases, your body can no longer regulate it's temperature, it overheats, you start to get light headed and eventually pass out. But, up until the passing out point, you can still recover by drinking fluids. Hospitals will just give you an IV of fluids just to speed up the process.
4. Various consumable things can cause dehydration. VG and PG are known desiccants. Caffeine causes dehydration. If you rarely consume caffeinated beverages, then drinking a highly caffeinated energy drink can result in dehydration. Otherwise, you will already be 'used to it' and consuming enough water to make up for it. Various Calcium types are also desiccants, including Calcium Carbonate (used as antacids). Salt and alcohol are as well, but I'm sure we all know of these two.
Sources.
Sorry, I'm not going to go digging for all this stuff online, those that want it can verify it themselves if they are truly interested in learning. It's just all kinds of stuff that I've learned from researching because I tend to not do well in hot weather. I sweat a ton and have trouble staying hydrated when doing anything physical if the temps are in the 80s. If it's in the 90s, I literally can't drink enough water to stay hydrated while doing a physical activity and have to take breaks due to feeling overheated and light headed.