Heat does indeed pass through a vacuum, Blobby. The vacuum of a vacuum flask prevents heat transfer by conduction and convection, but doesn't inhibit thermal radiation, which consists of photons in infrared wavelengths emitted from the source.
"Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero.[3] It represents a conversion of thermal energy into electromagnetic energy. Thermal energy results in kinetic energy in the random movements of atoms and molecules in matter. All matter with a temperature by definition is composed of particles which have kinetic energy, and which interact with each other. These atoms and molecules are composed of charged particles, i.e., protons and electrons and kinetic interactions among matter particles result in charge-acceleration and dipole-oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy away from the body through its surface boundary.
Electromagnetic radiation, or light, does not require the presence of matter to propagate and travels in the vacuum of space infinitely far if unobstructed."
http://en.wikipedia.org/wiki/Thermal_radiation
