Heat can be dissipated in a vacuum by electromagnetic radiation. This is how most heat is dissipated in a vacuum. For the Casimir interaction, virtual particle

Heat can be dissipated in a vacuum by electromagnetic radiation. This is how most heat is dissipated in a vacuum. For the Casimir interaction, virtual particles are one way to do the bookkeeping to calculate the force between two objects. One object transfers energy and momentum to virtual particles which in turn transfer the energy and momentum to another object.