Vpc to Vpc routing

Hi there - it is not possible to route between VPCs using a private vif. My advice would be to use Transit Gateway https://docs.aws.amazon.com/vpc/latest/tgw/what-is-transit-gateway.html. This is a routing device that can connect both direct connects and VPCs to each other. The other option as bwhaley pointed out would be to peer the VPCs to each other https://docs.aws.amazon.com/vpc/latest/peering/what-is-vpc-peering.html

What it is saying is that a single dedicated direct connect can be split into multiple logical connections knows as virtual interfaces (VIFs). In the setup listed on page 22 those VPCs would be able to communicate back to on-prem, but not to each other. That whitepaper looks quite old - I would use this one instead https://docs.aws.amazon.com/whitepapers/latest/aws-vpc-connectivity-options/network-to-amazon-vpc-connectivity-options.html

In short Yes, if you configure your on-premises router to support it.
Your router would just see them as two networks and send the traffic it received on one vif to the other vif.

However, why would you not peer them or use another technology in AWS.
The reason is sending traffic over your direct connect and then back to AWS will increase latency and costs.

If it is more that if your don't have routing in AWS will the direct enable it as a fall back then yes that would work.