If I use Grumpig's "Bench Manipulation" attack to have my opponent flip a coin for each of his or her Benched Pokemon, can I use Victini's "Victory Star" ability to force my opponent to re-flip those coins?

Yes, you can. If your opponent flips a coin as part of your Pokemon's attack, you can use "Victory Star" to force them to re-flip those coins.

Relevant cards

BWP_BW32