I have a possible solution! Here is my sorry tale:
I moved to California a while back. I noticed almost immediately that I was getting sick through alcohol way more than I ever had before. I chatted to a German girl who'd moved over at the same time and she had the same problem. She'd presumed it was the beer, but I was drinking G&T most of the time, so it was the same stuff I'd been having in the UK. I spent ages thinking about what on earth could be the reason for my sudden intolerance and finally came up with this:
I was in San Diego - perfect perfect weather over there. This meant that I was eating a lot less, especially in the evening, (seems that most of my food intake goes into keeping me warm) and very little fat. When I came back over here, I was a little horrified at the big lumps of butter that my mum was slinging around the place, but quickly got back into the swing of it. My intolerance for booze vanished (immediately - the weekend after I got back I went on a huge bender and woke up feeling wonderful). So.. you know what they say about a glass of milk before you go out. I reckon it must be because of the fat in it (surely the folk wisdom pre-dates skimmed milk). Fat makes your stomach retain its contents for longer (this is so that the fat is released slowly enough to be broken up, rather than just staying in one huge greasy lump). I reckon that whatever it is in those drinks that makes me vomit is also buggered up by the low pH in my stomach.
So do an experiment for me: eat a pound of lard and then go and drink eight pints of porter and tell me how you feel.
(By the way, the dehydration thing is what gives you the /headache/ (which I didn't get). I reckon it's something different that makes you sick)