A vacuum flask prevents conduction by making a vacuum between the inside and the outside layer of the flask, so that the only solids that touch are the lid and the main body of the flask. This means the amount of conduction that occurs is minimised. One major drawback though, is when the hot water transfers heat with the cold air inside the flask, then the hot air transfers heat with the cold lid, and then on the likely chance that the lid is hotter than the surroundings, the lid transfers heat with the air molecules around it. This means that a considerable amount of heat is lost to the surroundings.
The only way convection can occur is by the hot liquid or solid transferring heat with the trapped air inside the flask, although this only affects the temperature of the water by a bit, because most of the liquid condenses again.
The flask is made so that radiation is reflected back of the sides, which is made of a shiny material. Also, the flask is not transparent; it is made of an opaque material, usually plastic or metal. The only way radiation can escape, is by the lid being taken off.