Here's why:
* Vice Dragon's Summoning Condition: Vice Dragon's Special Summon condition states: "You can Special Summon this card (from your hand) if your opponent controls more monsters than you."
* The "More Than" Clause: The wording "more than" means you need to control at least one monster for the condition to be met.
* Zero Monsters: Since you control no monsters, you cannot fulfill the condition even if your opponent controls multiple monsters.
In short, you need at least one monster on your field for Vice Dragon's summoning condition to be met, regardless of how many monsters your opponent controls.