from PyQt4.QtCore import *
from qgis.core import *

# Loads Raster.
vrt_path = 'path/to/vrt/file.vrt'
vrt_info = QFileInfo(vrt_path)
vrt_name = vrt_info.baseName()
vrt_layer = QgsRasterLayer(vrt_path, vrt_name)
if not vrt_layer.isValid(): print 'Not valid.'

# Gets bounding box and buffers it with whatever value you pass in offset.
bbox = vrt_layer.extent()
buff_bbox = bbox.buffer(offset)

# Prints coordinates in console.
print buff_bbox.asWktPolygon()