Strong and energetic (8)
'strong and energetic' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vigorous that I've seen before include "Full of energy and strength" , "Strong, healthy" , "Powerful, energetic" , "Energetic, forceful" , "Strong and energetic" .)
(Other definitions for virile that I've seen before include "Robustly masculine" , "Potent" , "male" , "Masculine and macho" , "Manly, strong" .)