Vanilla is okey?

I have four cats three of them like each other,but one female cat and the other is a male cat they don't like each other. My questions is do I need to put the vanilla to the two of them or to the four cats. I want this two to work out what ever difference they have. The female was a stray cat.
Yolanda

I would put it on all four of them so they all smell the same. JMO

If the cats already know each other and have been living together already, the vanilla will make no difference. Perhaps instead, try and find out what is going on between these two. Are they both trying to be alpha? Does one annoy the other to the point where a fight starts? Something I would recommend is a Comfort Zone plug-in. It emits a pheremone that calms and destresses cats and it may help these two. You can read about it here.

The first time I brought her home she will pee in my cat's sofa and she chase him.That is why I let her go.But last winter she was sick and pregnant so I decide give her a second chance. She had her kittens and she is spay and neuter. I tried the introduction and is not working. Now when he sees her he start chasing her.Sometimes they are sleeping ,one in one room and the other in the next room nothing happen but when they woke up and she start moving and sees him she start hizzing and he wants to chase her...As you say maybe one of them tried to be the alpha, and if that is the case how can i get them like each other. Oh by the way i used the feliway in plug in.
Yolanda