Despite being unclear about the reason behind the change in the Vienna Government's stance, Thomas took action immediately.
At this juncture, if he did not personally get involved, the meeting would likely become a one-sided affair upon starting.
Don't assume that just because everyone does not wish to see the French expansion, countries will stand against the French by default.
The assassination case has already put the Kingdom of Sardinia in a disadvantageous position. Moreover, there has been no action from the Sardinian Government on punishing the culprits that the other countries want to see.
Perhaps the Sardinian Government sees the Carbonari as too powerful and too widespread to act against, believing the assassination was simply the impulsive act of a few overzealous teenagers without any mastermind behind it.
