Thermal imaging and night vision are two very different technologies. Thermal imaging detects the infrared energy emitted by different objects and turns that into an image. Night vision works much like the human eye does, but requires much less light to function.
Thermal imaging can work day or night, in light or complete darkness. It also allows you to see through obscurants like smoke, fog, dust, or sand. In contrast, night vision cannot work in daylight or in lit rooms, and cannot see through obscurants in the air.
While a night vision device can typically offer greater image clarity than a thermal imager, it will be susceptible to the same challenges that the human eye faces. Most notably, objects that are camouflaged will not typically be easier to see with night vision. In contrast, a thermal imager will still detect these objects just fine.
Another difference between the two is that thermal imagers can track residual heat. This means you can see footprints and handprints left behind from a person or animal. If you are wondering whether your pet has been getting on the couch while you are away, you can use thermal imaging to detect a warm spot left behind.