My daughter has vomitted two seperate times from colds. My pediatrician and I both agreed it was from the mucus. So did my husband and I. We found that when she drank something that would loosen the mucus, she would gag. She is a HUGE milk drinker. And a water drinker. When she does have a congested nose she refuses both because she says she is afraid of throwing up from her boogies. When she did vomit one of the times,  she refused everything to drink. By morning we had her in to her ped. for an emergency appointment for fear of dehydration. As soon as my daughter found out she would need to have blood work done to check her levels she panicked and drank 16 ounces of water in 5 minutes. By the time the doctor came in she was out of danger and perked right up asked for food and a popscicle. We also were tripleing her Cortef through the night. We had to give her acedeminophen (non-asprin) suppositories to bring her fever down because she then refused anything orally. She did manage to take her Cortef though. But like everyone should have on hand, I had the Solu-Cortef on hand and ready If she could not tolerate or take Cortef orally.  My daughter is four and is a salt waster. (she has the 21 hydroxlase deficiency) 

She did not have a fever the second time she got a cold and vomitted. As a matter of fact she was fever free and I was thinking she was past any danger (fever) with her cold virus. Then she was laying on her back and that was enough to gag her with the congestion and she sat up and vomitted. Of course after the vomitting came a fever. So I do think it is the congestion causing the vomitting. Just always make sure their levels are good.