Answer: the presence or absence of xylem and phloem
Explanation: The striking and single most important feature that distinguishes the vascular from the non-vascular plant is the presence of tubular tissues/vessels to carry water and food to all the different parts of the plant, in vascular plants. These vessels are the phloem and xylem. The phloem transports food, sugars and nutrients, manufactured in the shoot to storage sites in the plant and the xylem transports water and minerals taken up by the roots to where they are needed.
Although non-vascular plants may not have these vessels, many have evolved simpler tissues that are specialized for internal transport of water, minerals and nutrients.