Instead of Callcentric, you could probably port your Vonage number to Anveo. Their Personal Unlimited DIDs are only $2/mo.
However, note that (as with your present setup), you can't use SP1 to forward to your own GV number, because when you call with GV to your own number it assumes that you want to check your VM. You could use SP2, but then Anveo would charge you for the outbound leg ($0.01/min.; 30-second billing).
One free path would be to call from SP1 to a second GV account, set up to go immediately to voicemail. Unfortunately, you would then have a second box to check (though if you listen to all your messages via email, that may not be a problem). Another option is forwarding on no-answer from Callcentric to a free VoxOx iNum that forwards to your GV account, though for some complex reasons you probably can't get the original caller's ID on the GV message.