New – Access Resources in a VPC from Your Lambda Functions
aws.amazon.com/blogs/aws/new-access-resources-in-a-vpc-from-your-lambda-functions
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Your Lambda functions can now access Amazon Redshift data warehouses, Amazon ElastiCache clusters, Amazon Relational Database Service (RDS) instances, and service endpoints that are accessible only from within a particular VPC. In order to do this, you simply select one of your VPCs and identify the relevant subnets and security groups. Lambda uses this information to set up elastic network interfaces (ENIs) and private IP addresses (drawn from the subnet or subnets that you specified) so that your Lambda function has access to resources in the VPC.