Direct opposites:
* Weakly
* Lazily
* Feebly
* Passively
Words implying lack of effort:
* Halfheartedly
* Indifferently
* Listlessly
* Reluctantly
Words implying gentleness:
* Gently
* Carefully
* Softly
* Tenderly
The best antonym depends on the specific context of how "vigorously" is being used.