[Revox]

Yes. Because technically the sets of computer units on each respective router are receiving their IP addresses from a different gateway IP, which places them in a different physical network, and thus why you cannot see the other units from the Vonage router.

Internally, you should be messing with each router's LAN settings. What you need to keep in mind is to tweak each one's DHCP server settings so that your units don't automatically get assigned the same IP addresses, thus causing an IP address conflict on your network.

Here's what I mean:
Your linksys router's IP address will default to 192.168.1.1
It is setup to create a DHCP server for which all the units connected to it will automatically be assigned an IP address.
Usually Linksys routers will default to use the IP addresses in the range of 192.168.1.100 - 192.168.1.150
You should see that relatively close to the LAN address box on your initial configuration page.

Since I am not that familiar with your motorola unit, it may end up being easier to place the Linksys unit into the same gateway as the motorola unit, seeing as how you can dictate what IPs the Linksys will use for auto-assign purposes pretty easily. Although, because your motorola unit is physically plugged into the Linksys unit, it should just be a matter of changing it's DHCP server settings.

Anyway, what I'm getting at is that you want to make sure that your Motorola unit (or Linksys unit depending on which unit's settings will be easier to alter) is not going to want to use the same range of IP addresses when it comes to auto-assignment within your network. That will only cause more headaches.

The motorola unit should offer the same types of configuration as the Linksys unit does, I'm just not as familiar with them. Your call as to which you alter and which you leave the same.

But to recap, in a nutshell:
Either change the LAN IP of the Motorola unit to 192.168.1.XXX and make sure it doesn't auto-assign IPs to anything within the range of 192.168.1.100-150, OR change the LAN IP of the Linksys unit to 192.168.15.XXX and make sure it doesn't auto-assign IPs in the range of whatever the Moto unit is using. This will put both units inside the same gateway IP address, and allow each unit to speak to one another on the network, provided none of the units are trying to use an IP address that's already been assigned to something.

Good luck!