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.
Source: TPCi Rules Team (2012-01-12)

Relevant cards

Note: All applicable cards may not be listed below.
BW11_23
BW3_14
BW3_98
BWP_BW32
BWP_BW32
SM2_10
HGSS4_23