Vieira will come up against Harrison, who he managed at New York City FC, when Palace travel to Elland Road on Tuesday.
Speaking about Harrison's journey, Vieira said: "I'm really pleased with how well he has been doing since leaving the MLS.
"He's a really talented young player who just needed to find the right place to play and to challenge himself because his work ethic is one of the best in the game.
"He is always wanting to improve his commitment to the game. It's fantastic and working with him in New York was really enjoyable."
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