Network speed between 2 EC2 instances in VPC

The physical interface is at least gigabit, and known to be 10 gigabit on Amazon's "HPC" instance types.

However, that information is largely irrelevant. Amazon's infrastructure has layers of abstraction to provide the scalability and manageability required for an operation at this scale. The network latency between regular EC2 instances can be surprisingly high, and varies a great deal over time.

Since your VPN runs on top of the regular EC2 networks, you'll have to consider this variance. So you will need to benchmark this yourself, and keep the benchmarks running for a day or two while your EC2 instance is under load -- to get a feel for how latency & throughput look 'on an average day'.