Q: If I have multiple virtual networks in Windows Azure, can I have a virtual machine (VM) accessible to all virtual networks?

A: Each virtual network in Windows Azure can have a single site-to-site VPN configured that can link back to your on-premises environment.

If you therefore had multiple virtual networks in Windows Azure and had a VM in one of those virtual networks that you wanted to make accessible to all the virtual networks, you would need to have a site-to-site VPN from each virtual network back to your on-premises network, then route the traffic between the virtual networks using your on-premises network.

At this time, there's no way to route locally in Windows Azure between virtual networks, and a VM can only be part of one virtual network.