Union Government
(1) While the office of Chairman is vacant, or while the Vice-President is acting as or discharging the functions of President, the Deputy Chairman (or, if that office is also vacant, a member appointed by the President) performs the Chairman's duties. (2) During the Chairman's absence from a sitting, the Deputy Chairman, or a person determined by the Council's rules or by the Council itself, acts as Chairman.
This fills in the gaps for who presides over the Rajya Sabha if the Chairman (the Vice-President) is unavailable, whether due to a vacancy, acting as President, or simple absence from a particular sitting - the Deputy Chairman is first in line, with backup arrangements if even that office is vacant.
If the Vice-President is momentarily out of the chamber during a Rajya Sabha sitting, the Deputy Chairman (or another designated member) takes over presiding duties for that period.