The open house is sort of a big cattle call and it's important to attend if you want to get into Versant.
They do different start times throughout the day, so it breaks up the hundreds of applicants into 3-4 groups. There is usually an introduction from the different facilities before you get started.
There are tables set up for the different facilities. Each one has representatives for the different floors that are hiring -- generally the nurse manager. You're vying for face time with them. It kind of reminded me of a convention, where you're waiting around just to get a few minutes with someone at a table.
Have an "elevator speech" of your qualifications and interest in the facility/floor ready to go. You don't want to stammer and stutter when you finally do get to talk to the managers. Practice it ahead of time.
Bring 20-30 copies of your resume to give out. Business attire is appropriate. You need to present yourself very well to stand out among the applicants.
The day after the open house, email each and every person you talked to at the event. Keep it brief, but be sure to reference something specific from your conversation -- like if the oncology floor told you about their self-governance, mention that. It helps them remember who you are, and shows that you're interested.
Follow up! The hiring manager for one of the facilities told my class that he wanted to hear from us each and every week.
I'm not sure if you are ADN or BSN, but for the June 2012 cohort, we were told after we began the process that some facilities (UMCB, SNW) would not consider ADNs unless they were already employed at the facility. I'm not sure if they are continuing that policy for future cohorts.
Good luck! I interviewed for Versant but ultimately decided it wasn't for me. Seton is a great system, though, and I know many people who are happy there.