That's a big space to cover in 12 days. How are you thinking of getting from SF to Vegas? Assuming a flight, that's 4 flights in 12 days - 3 of which are quite long (potentially an extra one depending on if you get direct to SF).
I've not done that trip myself, but I'd recommend booking it yourself if that's the trip you're wanting. Flights, hotels, and excursions are so easily researched and booked online these days that you shouldnt need to pay a third party to do it. I've done similar multi-city breaks and would always just sort it all myself.