it is because there is more water vapor in the air, and water vapor is the strongest greenhouse gas
Yes. CO2 increases have raised daytime temperatures; which increases water vapor in the air. This increases night time temps.
"Nighttime warms faster because daytime warming helps make the air hold more moisture then that moisture helps trap the heat in at night, Gleason said."
Water vapor is a strong greenhouse gas, but it is also transient. Atmospheric CO2 is the core long term factor that controls temperatures. It's like a loop. More CO2 = higher temperatures = more water vapor. I gather you get higher dew point temperatures caused by noncondescenceable green house gases.
This might help:
"Although water vapor probably accounts for about 60% of the Earth’s greenhouse warming effect, water vapor does not control the Earth’s temperature. Instead, the amount of water vapor is controlled by the temperature."
...
"If a volume of air contains its maximum amount of water vapor and the temperature is decreased, some of the water vapor will condense to form liquid water ..."
"The greenhouse effect that has maintained the Earth’s temperature at a level warm enough for human civilization to develop over the past several millennia is controlled by non-condensable gases, mainly carbon dioxide, ...
"Because these gases are not condensable at atmospheric temperatures and pressures, the atmosphere can pack in much more of these gases ..."
The article notes this is an area of active research. A certain amount of water vapor and cloud cover could reflect sunlight and mitigate warming to some degree, we are figuring it out.
https://www.acs.org/content/acs/en/climatescience/climatesciencenarratives/its-water-vapor-not-the-co2.html