If you are using QGIS 2.18, you can use Reclassify values under SAGA in Processing toolbox
- Go to
Processing toolbox → SAGA → Raster tools → Reclassify values
- Select raster file that you want to classify
- Method: Simple table
- Lookup Table click on the
… a new window will open
In the example, the raster data were classified into six classes, so you need to change the values based desired range and output
Reclassified grid: write a name for the new classified raster data.
Please make sure that you have selected the correct projection when you created the slope percent to give you the desired output from reclassify tool.
In QGIS 3.4.1, there is a Reclassify by table tool which is located under Processing toolbox -> Raster analysis -> Reclassify by table.
Using the tool is almost same as the one explained for SAGA Reclassify tool.
After reclassify the data, you can use Vectorize tool to convert the raster into vector data. The Vectorize tool exists in both QGIS 2.18 and 3.4.