So, I don't think Van Pelt is Red John but I definitely think she is close to him or is an accomplice. Look at these quotes from various episodes

"The Red Barn": Regarding Visulaize..Grace: What's wrong with wanting to be the most powerful version of you you can be?

"The Crimson Ticket": Teresa: Women can strangle people.

Grace: Believe me I know I could.

"Ring Around The Rosie: To Linda Tibbs "Grace: If you quit now, you're going to miss the best part. Payback.

So she has always had family issues. During the time that Red John started she would be probably 4-6 or somewhere in there. Who's to say that she wasn't having a rough go of it when she was a kid. She needed people to be her "family" and eventually takes her battered and broken self to a Visualize type group and became a member. I know she sits all day because the actress playing her is/was pregnant. But she is one of the only ones who basically watches the incoming/outgoing workings of the group. She could easily put a head up to RJ or anyone connected. It might explain how he is always one step ahead. I just find her recent reactions strange. She is more of a fringe character these days and has a very "bad" girl side to her.

Side Note: This part is out there so don't judge it along with the above paragraph but we don't know where the smiley face on the barn came from. Our assumption is that Red John got his start doing that after he killed those 2 Visualize members. What if it was just some kids who wanted to paint the barn and RJ then copied it as his symbol. What if it was just a Visialize symbol in the early days? Who knows who would have come up with it.

Theorizing that Grace Van Pelt is a member of Visualize based on how she defended their philosophy in "The Red Barn". Red hair, Visualize member, her cousin's a "psychic", engaged to known Red John accomplice, joined CBI same time as Patrick... she is not Red John but it will pan out that she is involved, even if not directly (e.g. Red John's daughter).

I think van pelt is red john because sche is a man who was transformed into a woman. She also has red hair which fits the name and sche also has a relationship with the devil. Sche also tries to withstand jane a lot so which would imlply sche's got something to hide. 

Sorry for bad english im russian :D

In the very first episode Van Pelt angry asked Jane- "Do you believe in God?" I think that means something...

I believe GVP was a man (RJ) before some surgery. I think that the casting for her character was intentionally searching for "large shoulders women".
I would like to see pictures of the other casted women for that character.
(Sorry for my very bad english!)

I believe that Van Pelt is definatley connected to red john. There is a picture on the website screenshot.com/the mentalist season 5, which shows all the main 5 characters on it. Jane is sat on a couch flanked by cho and lisbon which shows they are his closest allies. rigsby and van pelt are stood up. rigsby is facing the 3 on the couch where as van pelt has her back to them looking at them in a mirror. I think this is a message from the writers to say that van pelt is not all she makes out to be.
I agree with many theories that van pelt has lost a family member due to Janes con artist scams and therefore is working with red john to destroy and hurt Jane.
I think she used O'loughlin and then was given orders to kill him by red john. O'loughlin was sent to the safe house by red john to be killed by van pelt.

I think that Van Pelt is connected to RJ somehow or maybe even her father is RJ. In the episode when the guy, who was killed in Mexico wrote "He is ma ..." he might ment Heisman, who was a famous football coach and his full name is John Heisman which gives us the connection with Grace's dad who is also a football coach. He is from Iowa where there is a team called Iowa Tigers which may be connected with the poem "Tiger, tiger". The only thing missing is that we don't know did they ever met with Jane. Plus it's strange that O'Laughlin knew her dad.