Did the Sultan ever send some private letters to Vladimir to humiliate him on the status of his brother? Or maybe some akward details from his past, which only his brother would have known?
So, this is something which we're not exactly sure actually happened. For some background: Vlad The Impaler's father, Vlad Dracul, was ousted from his position in 1436, but regained power 6 years later with the help of the Ottomans.
He was to pay them tribute and to send his two legitemate sons to live at the Ottoman court, Vlad III (The Impaler) and Radu. While Vlad did not enjoy his time at court, and later left under Ottoman auspices to inherit the throne from his later father, Radu enjoyed himself at court and became a favourite of the Sultan's son, Mehmed II (Also known as the Conqueror for conquering Constantinople) becoming a powerful vizier.
Some Greek sources tell us that they were intimate and that they spent "day and night together" both before and after Mehmed's accension to the throne. This is supposedly where Radu gets his namecel Frumos, or "The Handsome". He also supposedly converted to Islam. I say supposedly because we have conflicting sources on all of these. However, it is unlikely that the Sultan would have "taunted" him or otherwise mocked him with intimate details, considering the brothers actually fought numerous times!
Both Vlad and Radu were able commanders, and during Vlad's battles against the Ottomans, Radu was sent against him on numerous occasions as the commander of the Janissaries, eventually helping to chase him out of Wallachia and eventually being instated as Pasha of Wallachia. His death in 1475 and the resulting power vacuum is what triggered the return of his brother. If you have any questions feel free to ask!
I can't answer the question, but I'm very curious where you heard this?