Ex IST: (at outstations it will more depend on the local procedures/staff)

Most likely they will tell you that TK doesn't VDB Elites. Forget about trying to explain that you are prepared to do it voluntarily. There is a dedicated TK "Denied Boarding " office airside at the D gates where you can inquire. Otherwise ask at the gate. If VDB is needed senior TK staff will be walking around the gate area inquiring with groups of turks/arabs if interested in VDB. I might be wrong on this one, but my impression is that TK try to avoid doing VDB with foreigners/western people. I have no idea what they are offerred to accept VDB.

Based on my experience with a recent overbooked IST-CPH TK flight where several VDB were needed. There was a separte thread some time ago also about VDB; I posted in that as well. Try to look that up for more recent experiences,