My request is what is the role should i assigned to a domain user to perform all NSX tasks without any access to vCenter tasks or for minimum Read-Only to vCenter tasks.
Because as per my understanding from the below screenshot, if i give NSX admin it will be with R&W access to vCenter & SSO configuration. is that right ?
No, NSX roles are for permissions to NSX. vCenter and SSO configuration you are referring to is for pairing NSX Manager to vCenter in NSX Manager appliance GUI.
If you want to access to assign a user or a group to access NSX, you need to assign rights for the user/group to vCenter first.
Then only after that NSX can give permission to the user, as per this doc NSX assign a vCenter User: Assign a Role to a vCenter User
If the user does not exist or has not given a permission to vCenter, then you will not be able to give NSX permission to the user.
A minimum Read-Only access to vCenter is required.
If you want to give permission to a group, then you can review this doc: Group-Based Role Assignments