Is Preen safe as a vegetable garden fertilizer?

Preen is not a fertilizer. It only prevents seeds from sprouting. It prevents your vegetable seeds as well as the weed seeds it is intended to target. They sell an organic variety that is specifically designed to be safe on food crops. That version of Preen is safe to use. Since most vegetable gardens are tilled under annually manual weeding shouldn't present an undue burden and is more fiscally resposible. That said Preen for flower beds and Preen for lawns can both be great time savers. Specific questions about commercially available products can usually be ask of any reputable dealer. Very large garden centers often have an expert on hand during their busiest days.