And now from the old school and someone who had a soy baby, I half agree and disagree with the above.
We old school believe if your baby is hungry feed it. We from the old school do not believe that water should be withheld from a baby (bottled water, of course) because your body is 95% water. You figure it out.
My son was allergic to milk 32 years ago. And so were my grandsons, less than 5 years ago and 8 years ago.
I also have a son that was 10 pounds when he was born and wasn't allergic to anything. This son cried relentlessly in the hospital (when he was 2 days old!) until they gave him cereal in his bottle. Then, this son slept through the night with no problems.
The youngest, who was a soy baby, threw up as yours did, as well as the grandchildren (2 of them the 10-pound boy's sons). It took years to get one grandchild satisfied with one milk, even when he outgrew the bottle.
My Soy milk son became extremely butterball up until he was 2 years old and off the bottle.
My 10 pound son is now 6'7" tall and weights over 300 pounds. My Soy baby is 6' and weighs 170, if he is lucky.
My sons were both on extremely well-balanced diets throughout their formative years. It is genetics that made one son bigger than his father (height from my side, size from his) and the youngest skinny (from my side) and not-so tall (from his side) .
The moral and idea to my story is do not let your child go hungry because a doctor tells you to. If you feed the baby and he/she is satisfied, then you have succeeded. If he/she is not satisfied, then recheck with your doctor. If your baby becomes constipated because a doctor/nurse tells you not to give a baby water, give the baby bottled water or prepare to use a suppository. . . . which is not fun.
We from the old school ask this and the next generation of mothers to use your intuition - which is there, I promise you. Do what's best for your baby and your own peace of mind.
I'll get off my soap box now and hope you find a happy medium between the above answer and mine.