I want to know is there any diet precaution is required after doing Vaman Dhauti. Can I take tea after one hour?
I had to look this up, because I'd never heard of it. I recommend seeing an ayurvedic practitioner who knows about these things if you want to feel comfortable with exact protocol. If you are down to use your intuition, I'd say see how you feel after an hour... do you feel like drinking tea? Or are you doing so our of a daily habit? You'll certainly know after you drink it if it was a good idea. 
Normally vaman is cleansing kriya. It would be good to have very light food like semi liquid rice or soup rather than tea or coffee. Thanks
I use to do it in 1973 when I did Sivanada Yoga, but more recently just drank a 4 cup jug of warm salt water and let it pass through, not a regular practice but will have yogurt or probiotic after.