“why did Article 2 contain the “grandfather clause”? Why was it needed??”
The grandfather clause allowed naturalized citizens who had risked their lives for the Revolution to become President - men like Alexander Hamilton, who was part of the group who added it. From Justice Story:
“§ 1473. It is indispensable, too, that the president should be a natural born citizen of the United States; or a citizen at the adoption of the constitution, and for fourteen years before his election. This permission of a naturalized citizen to become president is an exception from the great fundamental policy of all governments, to exclude foreign influence from their executive councils and duties. It was doubtless introduced (for it has now become by lapse of time merely nominal, and will soon become wholly extinct) out of respect to those distinguished revolutionary patriots, who were born in a foreign land, and yet had entitled themselves to high honours in their adopted country. A positive exclusion of them from the office would have been unjust to their merits, and painful to their sensibilities. But the general propriety of the exclusion of foreigners, in common cases, will scarcely be doubted by any sound statesman. It cuts off all chances for ambitious foreigners, who might otherwise be intriguing for the office; and interposes a barrier against those corrupt interferences of foreign governments in executive elections, which have inflicted the most serious evils upon the elective monarchies of Europe. Germany, Poland, and even the pontificate of Rome, are sad, but instructive examples of the enduring mischiefs arising from this source.”
http://press-pubs.uchicago.edu/founders/documents/a2_1_5s2.html
“You can read it in the contemporaneous legal treatise written by Vattel The Law of Nations, and the Principles of Natural Law.”
No, one could not. Why? Because ‘natural born citizen’ is a mistranslation of “Les Naturels ou indigènes font ceux qui font nés dans le pays de Parens Citoyens”, and that translation was made in 1797, 10 years AFTER the Constitution was written. Prior to then, all copies used the more accurate “The natives, or indigenes”.