1. I've tended to see slightly better overall performance running multiple VMs (nodes)/server.

2. We use zookeeper and cache locally with an ETS wrapper. It's not-erlang but there are a lot of nice zookeeper tools and the ezk module for erlang is very solid.

3. we typically don't do vpn as if the vpn goes down it is a disaster. dual nics are ideal. i don't always configure that way due to limitations of the environment (non-VPC AWS, for instance). but what are you trying to control? access to the node even if you know the cookie?