OK, I have spent days trying to figure this out, but I have information to share that might be helpful to others.
First, I bought the OMNIA card/ROMA Pass. I know many people say it's not worth it, but it depends. I got the 10% discount so the cost dropped to EU 101.70, down from $113. For the Vatican Museums, St Peter's, Sistine Chapel and St John in the Lateran and the Cloister, the retail cost from the official site is EU 55.
The ROMA Pass that's included is EU $38.50 retail and I'm using it for public transportation (EU $18, I use it many times a day on tram, metro and buses) the Palazzo Barberini + Corsini Galleries (EU 12) and the Colosseum/Forum/Palatine Hill (EU 12) and the discounted rate to the Capitoline Museum (EU 9.50 discounted) at a minimum. So retail that would be EU 53.50. I know my math skills are poor so if you find an error, please leave me in blissful ignorance. ;)
Anyway, 55 + 54 = 109 EU I think. Small savings and I'd rather have two cards than a bunch of separate tickets, where everything else is more or less equal.
What I figured out today, which was bugging me, is how far in advance can I book my actual Vatican/Sistine/St Peters sessions or reservations or whatever you want to call it. By fooling around with the booking link that came in the email that was a receipt for the OMNIA Card, I can tell you that as of today you can book up to Dec 31 2018.
When you try to book your 3x sessions (or seats) too far in advance, this is the error message you're going to see - There aren't seats available for the selected date.
I'm visiting in May 2019, and since you can change your mind on days and times as much as you like using that link from the email, I wanted to book my preferred day since I know it. But no can do. I'll try again in Feb.
But if you are planning on visting Rome btw now and Dec. 31st and you buy the OMNIA Card (still 10% off today only), do book your seat/session. I saw lots of days that were already sold out.
Happy trails!
Susan