How is your cat's urine output ? Any straining or tinge in the urine? Have you noticed a decrease in output?
What is his typical food ?
Don't blame yourself for this - the soy oil isn't considered 'bad' for cats, in fact, many vets are recommending it for situations and individuals who need a boost to their Omega-3 Fatty Acids (take a look http://www.peteducation.com/article.cfm?cls=1&cat=1400&articleid=665 )
Give him a break with the water. Check a few things for me first ok?
1. Gently lift the scruff of his neck in your hand and hold for the count of 5
Let it go and count at the same pace how long it takes to return to normal
2. Take a look at his gums and let me know if they are his usual color
3. Describe the vomit/regurgitation. Was it a puddle or a tube shape?
OK, he's not dehydrated and probably is just experiencing some intestinal upset from too much, too fast. The oil is going be smelly on him for a while.
You should be safe just waiting and watching for now. Leave fresh water out for him, but don't force it.
The only thing that concerns me is that you mention he drinks a lot in a typical day. Cats aren't supposed to drink a lot. They're supposed to attain most of their hydration via their foods - so you may want to modify his diet overall to be mostly soft food and less kibble. www.catinfo.org is by a vet who specializes in feline nutrition and has no ulterior motives for her info (no sales). She also includes homemade recipes you can try.
By the way - I used to breed/raise Siamese - they are a special favorite of mine. Even now as an animal rescue, the on-premises permanent residents include two siamese hybrids (snowshoes).
If you notice any extreme lethargy, inability to get up/respond, loss of balance upon walking or inability to walk, a show of pain upon urination/defecation or cessation of urination (or any tinge of blood in it) - waste no time in getting vet treatment.
I wouldn't expect any of this, but keep an eye on him.
Again - you didn't do anything wrong. Check back and let me know.