Does requiring SSL matter if using AWS RDS and AWS EC2 in a VPC
This question -- and its answer -- are primarily regulatory/compliance matters.

If you need to comply with a standard that mandates encryption in transit between systems on the network, then you need SSL on RDS because the AWS network only isolates your VPC's traffic from the machines in all other VPCs -- the traffic is not actually encrypted on the wire as it traverses the cables, routers, and switches within the availability zone or region.

Traffic between systems in a VPC is still traveling unencrypted over the AWS infrastructure within the region, where it is theoretically possible for the traffic to be intercepted by a malicious actor who was able to compromise the AWS infrastructure. Such a compromise is considered highly unlikely, as there are protective controls in place such that it should almost necessarily require physical access and a conspiracy among multiple AWS insiders, but "highly unlikely" isn't the same as "impossible."