Peter Odemwingie put the Baggies ahead but Charles N'Zogbia and Ben Watson struck back before half-time only for Marc-Antoine Fortune to salvage a point for the home side.
Di Matteo said: "I think he (Vela) had a good start to the game, was dangerous and he will be a very good addition for us."
The Albion chief felt his side should have been awarded a penalty late in the game after Gary Caldwell appeared to handle the ball.
He said: "We didn't get a penalty again. It seems like every game now. We didn't get one in this game, against Blackburn, against Manchester United.
"They are adding up and they are costing us as well."