if you are AAA member, Hertz waived fee for 1 carseat.
I guess you will need a double stroller that can fit one bassinet, take the bassinet base with you, fly with the stroller check in, bassinet on the plane.. when you arrive, you will have one car seat waiting for you. and your own bassinet...
Uppa baby vista can use infant car seat with a stroller seat at the same time. You can try to look for a 2nd hand one. And you can use your own infant car for the 6months old all the time in vegas.
For car seat rental, some car rental company charge $10 per day. I just bring my own if the trip is more than a week or so.
will you rent a car at LV? u can rent carseat from the car rental, but make sure if they have infant carseat, and convertible carseat as yr kids are still too young.( normal they only have booster)
Stroller, you better bring your own, airlines don't count it as a luggage.