Trump was a candidate for the office on the day of the assassination attempt, not the office holder, so whether or not Vance proceeded to the top of the ticket for the November election, or another candidate for the office was selected prior to the election, and the necessity of either Vance or another candidate winning the election, would have to occur before Vance would have been president had the assassination attempt been successful.
So no, HA, you are not interpreting the situation correctly. Your interpretation would apply now, but not at the time either of the assassination attempts upon Trump were made.
Now that Vance is actually the VP, and constitutionally the first successor in the event the president is unable to execute the duties of his office (Biden excluded, of course) that's a safe bet.
