Enabling "Verified by Visa" through AEON?

I've regularly used my USA-issued VISA card with online vendors in Thailand. They have all used "Verified by Visa" through Kasikorn bank. After the first transaction where ISTR I had to register some info, for any new transactions I now am prompted for a password and the Verified by Visa service then processes and approves the transaction. I've done this dozens of times.

However, I'm now trying to use the same VISA card with another online Thailand vendor, and they us Verified by Visa through AEON, and all my attempts to use the VISA card are declined. I called the card issuer in the USA and they show nothing being declined, or even submitted (i.e. no transactions at all,) on their end and suggested it was a problem on the Thailand end.

I contacted the vendor in Thailand and he said "it looks like it could be because your card is not fully enabled for the Verified by Visa authentication, if it isn't then it will be rejected by their system before any transaction is attempted."

When using Verified by Visa through AEON I'm never prompted to register or log-in.

Anybody know how to become "fully enabled" with Verified by Visa through AEON?