Vitamin B12 can, for some, increase energy shortly after taking so it is often better to take B vitamins in the morning, in case the energy keeps you up at night.
Because it is sublingual and will be absorbed directly into your bloodstream it doesn't matter too much whether you take on an empty stomach or not. Some people feel nauseous taking B vitamins on an empty stomach though, so listen to your body and opt for what is best for you.