Mirrors do not absorb light (at the physical level we are discussing). Yes photons are lost after light hits any object (light bends even from the force of gravity) but that doesn't really effect how they work in a grow room. Mirrors do act very much like magnifying glasses - shine a bright light (like a surefire flashlight or an HID light) into a mirror and move the light (or mirror) around until the light beam hits you in the eyes. That is a hot spot. After the spots in your eyes go away - go read my earlier post. They differ from magnifiers in that magnifiers increase and focus the light/heat whereas mirrors just focus the light/heat (no increase).
the hot spots you get from mirrors are completely unrelated to what stage the plants are in - you can see a mirror "hot spot" anytime you want - try the "shine a light in a mirror" trick. But if you are asking can mirrors burn plants in veg stage then the answer is yes.
But if you don't believe me then go ahead and use them.