Vlaar knows West Ham United will look to exploit their defensive weakness although the Dutch international, who has played five games in 15 days since returning from injury, can see signs of improvement.
Vlaar said: "Are teams beginning to target Villa? People will know our struggles (at set pieces). I bet they do. That's what I would have done.
"West Ham have a very strong team and play a lot of long balls. We know that so we have to be ready for that.
"It's something we have to take care of and focus on. We know the situation, I'm not afraid of that. We have to stand up and be counted, be brave enough."