To know the answer lets see what happens with our food.
Right from your saliva in your mouth food began to digest.If we drink water after the meal, it dilutes the digestive enzymes, which weakens the digestive process.
In the ancient Indian text Arthashashtra , Chanakya mentioned not to drink water just after having the meal but after some time.When food gets digest, then one should take water.
Vajrasana is the only asana performed after the meal, as it helps to digest the food.When we fold our feet from knees and sit, it cut down the blood flow in our legs, so whole flow works inside the abdomen, which improves the digestive process.
So never take water after the meal .You can directly go to perform Vajrasana after the meal.