Do this first and tell us what the result is :
mySQL= "INSERT INTO veh_parts "
mySQL= mySQL & "(veh_id,part_name_id,part_num_id) "
mySQL= mySQL & "VALUES ('" & Session("veh_id") & "','"
mySQL= mySQL & Session("part_name_id") & "','"
mySQL= mySQL & Session("part_num_id") & "')"
response.write mySQL
response.end
myConnection.Execute mySQL
myConnection.Close
Set myConnection = Nothing
It will show what SQL you are using. Normally a data mismatch occurs when you try and put the wrong data type in a database column.
eg :
If veh_id is numeric in the DB then you use quotes like so :
mySQL= mySQL & "VALUES (" & Session("veh_id") & ",'"