If you're asking whether you can have your own vonage VOIP adapter (typically only available as a wired device) using your neighbor’s wireless network, the answer is yes. You just need a wireless ethernet bridge to convert that wired ethernet connection into a wireless client. You can either buy a standalone wireless ethernet bridge, or if the Mac has an available ethernet port, you could connect the VOIP adapter over wire to the Mac and bridge that to the Mac’s wireless, thus using the Mac itself as a wireless ethernet bridge.
Standalone wireless ethernet bridge
[neighbor’s wireless router]<-- wireless -->[wireless ethernet bridge]<-- wire -->[voip adapter]
Using Mac as a wireless ethernet bridge
[neighbor’s wireless router]<-- wireless -->[mac (wire and wireless bridged)]<-- wire -->[voip adapter]