Yes, you can use a variable ND filter and a polarizer together. However, there are a few things to keep in mind.
First, the ND filter will reduce the amount of light entering the camera, so you'll need to compensate by opening up the aperture or increasing the ISO.
Second, because the ND filter is reducing the amount of light, it will also slow down the shutter speed. This means that you'll need to use a tripod or some other form of stabilization when using this combination of filters.
Third, when using a variable ND filter, be sure to keep an eye on the exposure meter in your camera. The reason for this is that it's easy to accidentally overexpose your image when using a variable ND filter because you can't see what you're doing through the viewfinder.
Finally, remember that polarizers will darken your image, so again, you'll need to compensate by opening up the aperture or increasing the ISO.