Vieira told the Sunday Mirror of his France teammate: "He is a positive person and he can play in a number of positions - at fullback or in central defence.
"William has made a big contribution to that. He will have had a great influence on the younger players with the advice he has given them.
"He is a great character to have around the club - and a very important player for Arsene Wenger.
"He never gives up and will bring the best out in the young players around him. I have played against him and he is a very difficult opponent. I've played with him for France and he is a great player to have on your side. He has great experience and Arsenal will need that in the second half of the season.
"William has been a great signing for them. He is also happy there, you can see that."