If my opponent used Scyther's "Afterimage Strike" last turn, and on my turn I use an attack like "Double Slap" to flip two coins and do 30x the number of heads, if the opponent gets heads on "Afterimage Strike" to prevent the damage, can I use Victini's "Victory Star" Ability to reflip my coins for "Double Slap" and start the process all over again?

No, you cannot. Victory Star must be invoked prior to determining the amount of damage being done, which in turn is prior to the opponent performing the flip for Afterimage Strike.

Relevant cards

BWP_BW32