I recently was trying to help someone migrate from their Zyxel VMG3313-b10a a/vdsl modem to OpenWRT, and couldn't find any solution (apart from calling customer support) for extracting the pppoe username and password from the backup configuration file. These files commonly begin with <compressed alg=lzw len=XXX> <crc=0xyyyy> followed by a binary blob...
Once I figured out how to do it, I decided to write up my script (it's python). It can be found here:
Here's sample output:
$ python ./zyxel-extract.py configuration-backupsettings.conf
Users configured on router:
Username: root Password: 3peV!Nb1$54
Username: admin Password: turktelekom
PPP configuration:
Username: xyz824095721@ttnet Password: 6b88OF73
Wifi info:
SSID: 'My Wifi' Authentication: WPA(2)-PSK Password: 'mysecret123'
Theoretically it should work for any Zyxel modem/router, but it has only (currently) been tested on the VMG3313-b10a... if you find bugs, i'll be glad to fix them 
Enjoy,