By default Lambda is not on your VPC, that will need to be specified when you’re creating your lambda function and an elastic network interface will be created to connect your lambda to your VPC. Normally Lambda is normally deployed onto an AWS owned VPC. The reason you’d want to be able to connect it to your own VPC is the ability to connect it to perhaps an RDS database on your VPC or possibly a set of EC2 instances on that subnet. Now onto RDS and Lambda, when utilizing RDS for postgreSQL or Aurora MySQL you can actually invoke Lambda functions from within your RDS database, your database will need a connection to your lambda function and be in the same VPC. These functions can utilize information from within the database as it’s the database invoking the function in the first place.