Do we routinely cast
Disguise Weapon? If not, we should probably do so for this. Dark Sister would find a way to stab Viserys if he actually used another weapon, but she's somewhat recognizable. Even if they don't realize what blade she is from the start, Dark Sister is still made of Valyrian Steel.
Additionally, on the off chance we don't have a crit defense already,
Twisted Innards Is a cheap way to get a 75% chance to ignore the extra damage. Do we need it? Probably not. However, giving your organs the ability to dodge is hilarious, and the look on his face when Viserys just ignores a critical hit would be funny.