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We checked with both properties and here's the deal.
The Rio has motorized scooters available from the concierge desk. They're provided by an outside company, but they keep them on property, so you don't need to book in advance (although it wouldn't hurt). The cost starts at $40 for a single-day rental and goes up to $120 for 5-7 days.
The Palazzo and Venetian offer the same service, although here it's Guest Services you need to talk to. The same rates apply (no doubt it's the same company they use): $40/day, $70 for 2 days, $105 for 3-4 days, or $120 for 5-7.
You can find a heap of other useful information for mobility-impaired and other special-needs visitors in our Information for the Handicapped section in our dedicated transportation section ("Shuttles & Buses" in the top menu), while there's all kinds of other useful information and tips about hotels, gambling, and entertainment in the Special Needs section of the "LVA Visitor Guide.
If you use any of the transportation services when you're in town (buses, scooters, rental cars, taxis, etc.), don't forget to add your comments and ratings for the benefit of other mobility-impaired readers. Thanks!