If you want to make a material reflect cubemaps, you use the
$envmap property. Follow the link to the property's page on the VDC and you'll find other properties you can use to adjust the result. Specular masks are very useful for controlling spots of reflection, and their strength, across a material. A good example to study is
cp_manor/wood_floor01.vmt. Note that the
$reflectivity property controls the amount and colour of light that bounces off a material, and is not used to govern the reflection of cubemaps.
Another option for reflecting stuff is
func_reflective_glass, though it rarely has a use in a typical TF2 map.