11.5 and 12.2 use two fundamentally different security models which goes back to my comment about backwards compatibility probably not working too well (or supported). 11.5 used LHSecurity, and since 12.0 it uses NetWeaver's UME. Unless removing the credentials in the Virtual data server allows the pass through authentication for the 11.5 Trusted server to work, I don't know what other options you can consider. Direct database connection (perhaps less desirable) from the 12.2 server?
An XMLQuery calling the 11.5 server with http requests asking for Content-Type=text/xml (like at the end of the sample url's I provided) is an idea, and there are several options for this inside a transaction, but a bit tedious if you're intending to do complex queries or lots of parameters in the sql syntax.