Ryszek and Ryszu yes, but never Ryszku.
All three are acceptable. 'Rysiek' is technically a nominative, but it is often used in the role of a vocative. 'Ryśku' is the true vocative of 'Rysiek'', but indeed is rarely heard.
'Rysiu' is technically a vocative of 'Ryś'. It is used as the vocative of the name 'Rysiek' even though its proper vocative would be 'Ryśku'. On the other hand, 'Ryśku' is likely to be heard in conjunction with 'panie': 'panie Ryśku' rather than 'panie Rysiu'.
You may add 'Rychu' to the list which vocative is mostly used between male friends or colleauges among the working class.
To me all your declensions look correct.