People usually lose on average one pound per day of water fast. Some days you may lose 2 pounds, other days you may not lose any, but on average, your weight may go down by 20 pounds per 20 days.
The more overweight you are , the more you may lose. Slim people do not lose as much as overweight people.
One part of weight loss is a loss of water. Once people start eating after a water fast, they usually gain back several pounds, whis is usually just a lot of water they lost due to dehydration effect of the water fast.
In order to lose weight on the plant based diet (vegan diet or ovo-vegetarian diet or lacto-ovo-vegetarian), you may need to eliminate all grains and all processed sugars from your diet, but that depends from person to person.
Some people loose weight even when consuming some grains.