The definition of the vpc_zone_identifier parameter is a list of subnet IDs to launch resources in. Subnets automatically determine which availability zones the group will reside.
So suppose I list eu-west-1a and eu-west-1c for that parameter and a desired capacity of 3. Is my ASG going to deploy my desired capacity randomly across the AZs (e.g. 2 + 1) or it will deploy 3 per AZ?
There will be only 3 instances distributed across the two AZs if the selected AZs have enough capacity. AWS tries to prioritize high-availability, so it will try to place the instances evenly across the AZs (2+1 in your case). Exact details are:
Amazon EC2 Auto Scaling attempts to distribute instances evenly between the Availability Zones that are enabled for your Auto Scaling group. Amazon EC2 Auto Scaling does this by attempting to launch new instances in the Availability Zone with the fewest instances. If the attempt fails, however, Amazon EC2 Auto Scaling attempts to launch the instances in another Availability Zone until it succeeds. For Auto Scaling groups in a VPC, if there are multiple subnets in an Availability Zone, Amazon EC2 Auto Scaling selects a subnet from the Availability Zone at random.
