Vatican Museum tickets www.museivaticani.va/content/museivaticani/en/organizza-visita/tariffe-e-biglietti.html
Buy your Vatican Museum tickets in advance on the actual Vatican museum website otherwise you'll have to just turn up on the day and joint the longer line for those who haven't prepaid for a ticket. It's 20 euros entrance fee if you just get one on the day and if you prepay online, it's only an extra 5 euros. On the Vatican Museum website itself they do refer those prepay online tickets as "Skip The Line". Be aware, though, even with the official Museum onlne tickets alll this means is that you already have your ticket - EVERYONE still has to queue in the long security line to get your bags checked whether you have prepaid for a ticket for not. You can also pay just a bit extra and get a ticket with a tour guide to show you round (small charge for headphones) - again, direct with the Museum itself.
Be even more aware, there are many unofficial ticket sellers online which often look like the main official website, often through Tripadvisor, that are also selling what they also call "Skip the Line/Early entry Vatican Museum tickets". They try to market these as saves you having to wait in a huge queue, or VIP type tickets, not mentioning the fact that the security queue is still a long wait and there is no jumping to the front of that for anyone. And they charge a huge amount, and make people believe they are paying for privileged entry. It's a load of bollocks and you will feel ripped off. You will pay a fortune to just meet a tour guide outside, you will then join a long security line of people who have also pre-paid online (either cheaply, direct with the Museum itself, or through the rip off merchant 3rd parties). Once inside, your tour guide will tell your group to wait a minute while she goes and gets the actual tickets for you all, and that's all you''re paying all that money for, for someone else to go and get your paper ticket at the (quiet) kiosk inside.
There is no privileged entry with the more expensive prepaid third party tickets like they try and make out. It works just the same as a prepaid ticket direct with the Museum, only way more expensive. So don't make the same mistake we did. But DO definitely buy ahead online - just ensure that you are doing it direct with the official Vatican Museum website.
I think we had the same issue with the Colosseum. I just left it too late to prebook direct with the actual venues so ended having to do the third party vendors. Won't make that mistake again.