Ilian Iliev gave a partial answer in this discussion - https://communities.vmware.com/thread/605658
"XaaS/vRA communicates with vRO via vRO REST API."
I can't seem to mention the user in any post, so I sent message asking for documentation and or more details to verify.
Actually its not exactly what i'm looking for.
The idea is to Externally Validate (via vRO actions) the form inputs, just as you can do it with custom forms :smileyplain:
I want the user to be alerted at the form step and not afterwards so he could make the change immediate.