Weird to think vacuum has a “temperature.” 4 Kelvins.
I don’t know about “virtual particles” but the tiny amount of energy in “vacuum” is from matter and antimatter pairs that appear (WHERE FROM is the Nobel question) annihilate each other and give off tiny amounts of energy.
Even the best vacuum has gas particles, too. Antimatter’s been found near the galactic center. (The radiation signature from the matter-antimatter annihilation is distinctive.)