Plenty of the hotels have heated pools off season:

Bally's - heated to 80F, but no lifeguards and no service

Bellagio, - open, heated, but restricted hours

Caesars Palace - open, heated to 78F, near high season services

Hilton - 8am- 8pm, heated

Luxor - 9am - 5pm, heated to 82F; no snack bar but allowed to take snacks/drinks from hotel

MGM - Grand pool open year round, heated to 75F

Mirage -open year round, 8am - 7pm, heated to 78/82F (jacuzzi to 102F) Closes for about a month in December for maintenance

Paris - open 8am - 4pm, heated, loungers and chairs out, but no service

Rio- always 1 pool open 8am - 5pm, heated, with normal service

TI - open year round, subject to weather conditions

Tropicana - glass partition used to separate covered section of main pool

Venetian - one pool open and heated, but no services

Hope this helps!