can sudarshana homam be conducted on the day of grihapravesham after the vastu homam

It is a not a good practice
because practically it is very difficult.
Sudharsana Homam requires very very madi and acharam and requires minimum six hours
of time and madi thaligai for homam.
Definitely at the time of Grihapravesam the house work would not be completed 100%
After Sudhrasana homam no - constructions, alterations, outsiders interference should be there
to get the full phalan of Sudharsana Homam.

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Also, by practical, Grihapravesam requires a very good muhurtha day but
Sudharsana homam does not requires.
Vadhyars and other colleges will have lot of works on a muhurtha day
and it is difficult to get good set of persons for homam.

The only advantage, the expense may be reduced and a half a day time also can be saved.

Logically doing Suhdarsana Homam after a Grihapravesa Homam can be considered as Anticlimax.

These are all my opinion and I will explain all these things to my customers,
even after knowing these, if requested I will do for their satisfaction.