Here are a couple of things to be aware of when using Auto Scaling, VPC, and ELB with Auto Scaling:
1. There is a limitation on ELB in regards to AZs and subnets. You can only have one subnet per AZ. You will most likely be creating an ELB (and using VPC) when using Auto Scaling so it is important to understand this.
2. When creating an autoscaling group (as-create-auto-scaling-group), make sure to specific the VPC subnets and AZs if you are using . The VPC subnets need to be in the AZs specified in the AZ parameter for the as-create-auto-scaling-group command. No checking is done to make sure this is true so it is up to you to make sure these AZs actually exist in the subnets you specify.