General antonyms:
* Weakly
* Lazily
* Passively
* Indolently
* Sluggishly
* Halfheartedly
Antonyms depending on context:
* Gently (if referring to physical action)
* Moderately (if referring to intensity)
* Slowly (if referring to speed)
* Calmly (if referring to emotion)
The best antonym will depend on the specific context in which you are using the word "vigorously."