If the air temp when I'm pouring concrete (small staircase) is over 90°F, if I cover the surface with (4 mil?) vapor barrier after finishing, will that be effective to slow down drying?
Yes, but it could also mess up the surface finish.
Instead, consider dampening the concrete with water to keep it moist and cool. A garden sprinkler applied periodically will be ideal.
Another option is to do the pour very early in the morning when it is still cooler. By 10 or 11 AM you should be done.
Also a sunshade/tent fly over the concrete will help cool it and extend the curing time.