You don't have to use a suspended unit. You can get low-profile sheet metal floor duct deflectors that divert the heat coming from the floor grate by 90 degrees so that it blows out along the floor. These fit in the kick space below most vanity units then you cut an opening in the front of the kick space cover to mount a duct vent faceplate. When I remodeled the bath in one of my houses I did that and it worked really well -- I loved having the heat blow across the floor onto my feet on cold mornings. If you do decide to go for the suspended vanity, the bottom of it will kind of do the same thing, diverting the heat flow onto your feet. The drawback will be that moisture and dust will collect in the heat register underneath. I think having a duct extension sealed inside a floor mount vanity is less of a maintenance hassle.
