Delving into PBX & VOIP bits for a bit now and just got a SPA941 to play with today. I jumped online to vonage and activated a softphone account for testing purposes and went to work.
To get the vonage account accessible through the phone you really only need to set a couple parameters:

Plunk the vonage server sphone.vopr.vonage.net into the proxy address, your phone number as your user and the password they provide as the password. Save and you’re running!

Things that don’t work: Vonage fun keys (* keys), which unfortunately includes voicemail. It tells me I have a voicemail but no combination of things lets me GET the voicemail. Edit your dialplan to be this: (*xxx|*xx|[3469]11|0|00|[2-9]xxxxxx|1xxx[2-9]xxxxxxS0|xxxxxxxxxxxx.) in the EXT 1 ADVANCED page and add *123 as your voicemail number in the phone page and you’re golden!

worth noting that the name you put in or the caller ID outgoing in other programs/phones shows up for other Vonage customers but not anyone else. (BOO VONAGE!) So put “King of the World” If you want to see how many of your friends have Vonage..

next is a dedicated PBX I think. Then I might go with a SIP service other than Vonage.