seems pretty clear cut to me not sure why other are splitting hairs.
Witch A Hex Fortune on Fighter B
Lets assume they're both 8th level.
Fighter B is now fortuned for 2 rounds.
Here's the key line "The target can call upon this good luck once per round"
So lets say after Witch A , a monster fires off an ability at the Fighter B with a reflex save
next the Fighter B tries to tumble through an enemy square and then makes a vital strike against another monster.
The Fighter may have chosen to reroll any one of the 3 checks he was forced to make during the round.since you see your result first and then take the better result you'd normally save the ability when you think you've failed the check. Or you save the luck for a roll you know is going to be tough e.g. the attack roll vs High AC Golem.
Round Ends
Witch A goes again using her move action she cackles and the Fighter is again covered for 2 rounds
etc
etc