It's not a firewall rule you have to setup to make your adapter use always the same connection, you have to manually create a NAT Table for this IP address, see the NAT configuration page and the "Outgoing" tab.
This is probably your problem, the load balancing try to swing the connection from WAN to WAN, but get blocked by the firewall 2/3 of the time.
You shouldn't have to create firewall rule to allow communication to Vonage, as long as you have a rule that Allow any connection from lan to WAN.
Give the outgoing NAT a try, I can't help you more than than I never played with outgoing NAT I always used Automatic, because I only have one WAN.