Fan of knives is really weak and vanish is very high cost and removes the Rogue's minions as well. Although you can clear the board it is very difficult to really do anything significant with the card. If you are at a point where you can play another minion after vanish would probably not improve your situation, but would be more of a stall.
Fan of knives is an awkward card since it only does 1 damage. At 3 cost it will most likely not kill more than 1 target turns 3-5 and will take up most of the mana those turns not giving the player any significant gain. After turn 5 this card becomes basically a 3 mana draw card.
For those reasons I don't carry either in my deck. My suggestion would be to leave vanish as it is but make fan of knives 4 cost but draw a card for each target hit.