Deploying application in 2 different VPCs Vs Deploying in two AZs in single VPC

Deploying application in 2 different VPCs Vs Deploying in two AZs in single VPC

It is always recommended we deploy application across multiple AZs within same VPC instead of deploying in multiple VPCs itself. Below are 2 important points,

1) When you deploy application in two AZs(1a and 1b) in same VPC, AWS will make sure those AZs (1a and 2b) are deployed in 2 separate data centers.
2) When you deploy application in two VPCs
           VPC1 --> AZ 1a
           VPC2 --> AZ 1b

There is no guarantee that 1a from VPC1 and 1b from VPC2 are deployed in separate data centers. It is completely automated process from AWS to pick data center when you select AZ. So possible that both 1a and 1b are deployed on to same physical data center. 

For example, AWS has three AZs in Ireland region (i.e. 3 data centers). so 1a, 1b and 1c not always mapped to fixed data center. For one customer (or VPC) 1a can be data center1 and for other customer it may be 1b for the same data center. 

AWS has done with for security and better utilization of their data centers. Most of the cases customer do select 1a and 1b, if they are always mapped to fixed data centers, then data center 1 and data center 2 will always be utilized more and data center 3 might be under-utilized. To balance load across data centers, AWS has done this automated process.

***This is not documented officially in any of AWS documentation, it is said by AWS Solution Architects and experience talks.