my dd (2.7) has this was sick solidly for 7 days. Everything came up, even the tiniest bit of water. Mix a sachet of diorylite with every drink, and try and make each drink sugary (flat cola is good) as it injects a bit of energy into the body, and even the smallest bit of energy aids digestion. dd was so lethargic, and like Molk said, didn't speak just cried when she wanted something and slept lots, or just stared into space.
sadly for us the vomiting quickly turned to diarrhoea and we are now on day 3 of diarrhoea with occasional vomiting. d is better in herself though, and at least is eating now. She has lost a huge amount of weight though, she is just skin and bones
The doctors weren't incredibly helpful TBH. They just check for dehydration and if your child is dehydrated they just send you home again. The worry is when they start to dehydrate as they can go downholl ver quickly - watch out for dry mouth and lips, no tears when crying, sin that doesn't ping back when pinched (particularly on the chest area) , fingers and toes that stay white for more than a second or two when squeezed, oh and not passing urine for loooong amounts of time, and it being very dark in colour.
On day 3 of the sickness dd had brown/orange urine, and only went every 7 hours or so, and she still wasn't dehydratd, so they are resilient little souls.
Only you know your child though, and if you are at all concerned, call the doctors. We had two check ups by the doctor and spent a couple of hours in A&E on about day 6.
Hope your little one gets over it quicker than mine