Kristin I read Omens quite some time ago. I had some trouble at first finding my way in this book but then remembered some plot points from the first book and it read smoothly. There are not a lot of references to the plot of the first book. So, while I would say it is not absolutely necessary to read Omens first, I think you would benefit from it.
Bobby Karmakar I don't think you have to. I read Omens before Visions but I'm saying I don't think you have to because my friend and my sister are both intrigued by Visions, without reading Omens. My friend in particular picked up my copy and opened where my bookmark was, the middle, and she couldn't put it down. She's excited to read her own copy again this holiday and then Omens. And Kelley also said in her series' in particular, she has to provide enough backstory, should someone pickup Visions and not know it's second in a series. Also for those who read Omens when it was first published and then Visions as soon as that came out, like myself :)
Judy I agree with arch dandy. this is one case where you do need to read the first book to catch a lot of what goes on in the second one.
archdandy Yes, you definitely need to read the first in the series before beginning Visions. There's a central mystery that gets unraveled throughout both books, and I assume the third one that's to be released as well.