Here is an example of opening a disk, and writting it out to a file.:
from vixDiskLib import VixDisk, VixDiskLib_SectorSize, VixDiskOpenFlags, VixCredentials
creds = VixCredentials("vcenter.domain.com", "myusername", "mysecretpassword")
diskLib = VixDisk(vmxSpec, creds)
diskLib.connect(snapshotRef, readonly=True)
diskLib.open(disk.filename)
info = diskLib.info()
...
metadata = diskLib.getMetadata()
...
fd = open("out.vdmk", 'w')
for i in xrange(info['blocks']):
buffer = diskLib.read(i)
...
fd.write(buffer.data)
fd.close()
diskLib.close()
diskLib.disconnect()