I don't know the exact percentage, but it's pretty high. I'm not sure if it's 100%, but it might be. That's why I try not to trade when the other person's void isn't clear ? ▽ ` )ノ
I don't know if this will help but I've been involved in approximately 200 villager transactions and I've had about a dozen voided villagers move into my cycling towns from those transactions.In fact,I had a voided Big Top move in just today.So that's around a six or seven percent chance of getting a voided villager.