The fact that the atom is in a vacuum makes no difference. In fact, all atoms are in local vacuums; air is made of molecules as well, remember, and in the space between molecules there is nothing (sort of). The concept of vacuum versus atmosphere only makes sense on a macroscopic scale. So, the atom will become infinitely massive as it is accelerated and so cannot be accelerated to the speed of light, although it can come arbitrarily close, because to do so would require that it be imbued with an infinite amount of energy.