A flat solar collector plate is a container that is covered with a glass plate. A pipe runs through this container. This pipe contains a liquid. This liquid heats up and flows to the boiler where the heat is stored.
A vacuum tube solar collector is best presented as a glass thermos flask. The sunlight falls through the outer and inner tubes and heats the water. Because of the vacuum between the tubes, the inner tube is very well insulated and the heat loss is minimal.
For this reason, the efficiency of a vacuum tube solar collector is always higher than the efficiency of a flat plate solar collector.