Docker for AWS CE existing vs new VPC

When I build a swarm using Docker for AWS CE and I build it in a new VPC, I am able to ssh into the various nodes (managers and workers). However, when I try to build into an existing VPC, I cannot ssh into any nodes. I have filled in all the required input but I think the networking is getting crossed up by the VPCs CIDR. My existing VPC is a Can anyone point out to me why this might be an issue?