Viator tours - Best for major sites

When are you going and how many days in Rome? Viator is not a tour operator, they are a reseller, and if it's more than whatever months out to book directly, they're just hoping to score tickets and resell them to you at a significant markup. Better to go directly, somewhere on each tour mentioned in Viator or Trip Advisor it does state name of tour operator, contact them directly. Avoiding crowds at the Vatican Museum is very unlikely. But you could see if there are still any early entry available, or go very late in the afternoon, some people say the early tours before 8AM opening have stopped but haven't seen the evidence. Tour Guy is reputable, booking directly with Vatican is cheapest and sometimes has some perks. The Search bar above is your friend. Try 'Vatican tour Forum' look at last 6 months. Good luck!

Understand Viator is a re-seller, they contract with existing tour operators and sell the tickets to customers. Upon meeting your guide you'll discover they work for a company that is not Viator. Quality and details of the tour can vary. Similar to how Uber & Lyft are simply service-sellers for independent drivers for hire.

As for avoiding crowds...you're going to Rome and visiting the Vatican. There will be crowds, a LOT of crowds, you have the ENTIRE world descending there to visit these places, crowds are a fact of life there.

The best way to minimize that amount of crowds to deal with are tours where you can visit either early in the AM or, PM.

I've had very good success with Walks of Italy, all the tours we've taken with them were less than 10-people and each of the guides were quite knowledgable and accommodating to everyone's questions & concerns. The Tour Guy is another popular guide service. RS Guides also do guiding outside of their work with RS Tours, you can find them here and see if they offer services when they aren't tied-up with a RS Tour.

When is your trip?
I believe tickets for both of those venues become available at 30 days out Rome time
So if you are inside that time frame your best bet is to book a guided tour if possible
Book with the actual tour company-not Viator

Walks of Italy
Through Eternity
Roman Guy

Are a few often recommended here

If you are still more than 30 days out then search this forum for info on how and where to obtain tickets yourself if you are up to it!

Otherwise book a tour with one of the above

I bet Mr. Steves has recommends in his guide

Good to know information!
We are well outside of the 30 days - trip is in late September.
But, meanwhile I'll check out those tour groups - and ordering Rick Steves books on Rome!
Thanks much!

Viator takes about 30% off the top, and subcontracts with a local vendor. This per a tour guide we used recently in Belgium.
Have a great trip.