A couple of quick things...
I have been able to send and receive faxes on my standard Vonage lines... I hear it is hit or miss, but it has worked every time for me. The difference between the dedicated fax line on Vonage is that it is just optimized for FAX/Data instead of voice. The second port on the PAP2 adapter is designed for fax with Vonage.
I am running QOS with my LinkSys WRT54G router, this allows me to give my Vonage line higher priority over everything else on my network, this allows for better quality voice conversations.
You can also adjust the voice quality with Vonage from their website, the 90kbs is the highest setting.