Please keep in mind teo things
1, we can't validate user inputs, before REST APi request
2, custom form or XaaS form dynamic data population will not work REST API.
May be try this one
Create a vRO workflow with all user inputs, only for REST API calls.
Task 1 : validate all user inputs with your business logic ( based on application, location , datacenter, OS ) [ throw error, if required send an email ]
Task 2: request vRA catalog item with user inputs
In above process, you we will not see any error if vRA portal incase user missed in the data. It will failed in test 1 itself