Water which has been used to boil veg is full of nutrients which the plants will love. A good source of potassium for spuds, toms, beans etc.
Apparently, wrt soap, it all depends upon which soap you use. Soft soaps, and insecticidal soaps are often used to treat plants, but some hand soaps and some dish wash detergents contain things which you would not want to come into contact with plant. Best dilute well, and ensure you do not wet the leaves or stems when watering.
I use a drop of'Fairy' in a pumped pressure spray to blast bugs off, but it is far more dilute than would be found in dish wash water.
'Grey water' harvesting systems, usually collect bathroom water, but not kitchen water.