Necropian Vampiress vs Chant of Rage

[Borg]

Example : Vampiress has been targeted with CoR.
Target is in adjacent zone, separated by non-passable wall.

Q : must the Vampiress activate her Flying ability so she can attack her target across the wall or can she choose not to gain flying and move around the wall ?

RAW seems to indicate she must do everything she can to attack target, thus spend 1 mana to gain Flying.

[Kelanen]

No. You are not forced to use optional abilities.

[Zuberi]

You may want to review the supplement section on Mandatory Actions. "the controller of the creature is not required to do anything beforehand that would make the action possible."

[Beldin]

Agreed. In this instance the chant of rage would NOT make you use optional abilities to cross the wall. Therefore the creature would be free to act as you wish it to, as you cannot fulfil the mandatory action.