Manuel,
Can you confirm for me, is the Opentext VIM Portal a separate instance, or is this a component installed into an SAP Enterprise Portal? I had been thinking this was a separate instance, a separate server, but after digging around I don't think this is the case.
If it's a software component installed into an SAP portal, then you don't actually configure SPNego for VIM; you just configure it for your Enterprise Portal, i.e. for the NetWeaver AS Java that contains your portal.
Whether you configure your user store to be the Java UME, the ABAP datasource, or the LDAP datasource is relatively immaterial for purposes of configuring SPNego; it will work with all three, as long as you correctly configure reference user mapping. If you choose LDAP as the datasource, and ensure that the user accounts have matching names between Active Directory and your ECC system, then you will probably have the easiest time of it. There is lots of documentation in SAP Help and all over SCN about configuring LDAP as a user store for AS Java, so it shouldn't be hard to find that with a quick search.
The SPNego configuration itself should be exactly as described in either Andy's or my own blog (Andy's is closer to your release version, however, so go with his).
If, after configuring SPNego, you find that SSO works fine when users first access the portal, but then they are asked to login again when they navigate to an iView that talks to the backend ECC system, that indicates that the SSO trust between the portal and ECC is not correctly configured, or the reference user mapping between the Java UME and ABAP datasource is not correct. Is this the symptom you're seeing? Or is the initial access of the portal still prompting for a login, right away?
--Matt