You can use
r.recode to extract the areas above a certain altitude, and then
r.to.vect to convert the resulting raster layer to a vector layer. The steps:
Import the data
You can import the data in a grass gis database using r.in.gdal or using the 'import raster data' menu (for more details see the grass wiki page Import of original SRTM tiles in HGT format.
Extract the area of interest
Next, you have various options to 'extract' the areas above a certain altitude, including the r.mapcalc, r.reclass and r.recode. In the example below, the r.recode function is used to reclassify all cells of the elevation map with a altitude > 120 to 1. The rest will be assigned a NULL (nodata) value.
Note that the field marked with the blue asterisk will be automatically filled in (it is the path to a temporary file) when you enter the recode rule in the field below.
Convert from raster to vector
You can now convert the resulting raster layer to vector layer with the r.to.vect function.
Or all in once
You can use the steps above on the command line in one go:
r.recode input=elevation output=highareas rules=- << EOF
r.to.vect -s input=elevation@PERMANENT output=highareas type=area
And the result
This gives you a vector map of all areas with an altitude > 120.