General antonyms:
* Weak: This is a very broad antonym that simply means lacking in strength or force.
* Feeble: Similar to weak, but often implies a greater degree of frailty.
* Listless: Lacking in energy or enthusiasm.
* Enervated: Deprived of strength or vitality.
* Sluggish: Slow-moving and lacking energy.
Context-specific antonyms:
* Gentle: If "vigorous" describes a physical activity, "gentle" might be the opposite.
* Passive: If "vigorous" describes a person's personality, "passive" could be the antonym.
* Inactive: If "vigorous" describes a state of being, "inactive" might be the opposite.
* Mild: If "vigorous" describes intensity, "mild" could be the antonym.
To determine the best antonym, consider the specific context in which "vigorous" is used.