Hi Manuel,
In my organization we use the ECC system as an ABAP user store for the UME in our Portal. If I were doing it over today, I might choose LDAP datasource instead, but it works fine with the ABAP datasource, I would fully expect it to work with the Java UME datasource as well.
In this case, SPNego is being used to authenticate your domain user to your Java engine. From there, you may need to ensure that you have SSO tickets correctly configured between the Java engine and other systems, such as your VIM portal. For instance, this is a standard part of setting up a Java Portal against an ABAP backend system, involving creating the SAPJSF user in the ABAP system and configuring that in the UME, and creating a SAPLogonTicketKeypair in the portal and importing that into STRUSTSSO2 in the ABAP system (plus a couple of ABAP profile parameters relating to accepting logon tickets). This establishes trust between the portal and the ABAP system, so that SSO can occur between the two.
So, my guess at this point is that you probably need to configure that SSO trust from your Java engine to your VIM portal.
Cheers,
Matt