I figured it out. Apparently, there are two sets of keys - inbound and outbound. Each have a shared secret. Unfortunately, after I initially set up and configured the Outbound keys, the dashboard did not allow me to view them ever again. So I had to use Visa's "Key Management API" to list out all of my account keys.
The Javscript V.init uses the Inbound API Key, and the Outbound API Key. On your backend, you utilize the Outbound shared secret when decrypting the payload.