But, I wondered why Vito sought a deferment for Michael from the draft. Inductions had only started about a year before, and they were quite limited. If I remember correctly, as a college student he would have had a deferment without Vito's intervention.
Olivant, are you really old enough to remember WWII draft procedures?

I'm not, but I've done research on conscription before. Congress passed a draft law in 1940 that had men selected by lottery for a one-year term. It was extended in '41 and called for an 18-month tour of duty. College students could be deferred, as they were in the Vietnam era, but that was strictly temporary--the man could (and would be) drafted as soon as he graduated. After Pearl Harbor, men as old as 45, including fathers, were drafted.
The dialog in that scene about Vito having pulled strings to get Michael exempted from the draft was done for dramatic effect, to show Vito's influence and Michael's independence. But, logically, Vito would have seen that war was likely, and would have taken steps to keep his boys out. Even if he had no plans to bring Michael into the olive oil business, he wouldn't want him risking his life for strangers.