Will a 40 pound weight vest improve my cardio?

Yes. When I was still playing HS football (3 yrs ago), I went to a sports performance center here in central florida and one of the things they had me do was get into a very high intensity mat routine wearing a weighted vest...i went thru a 6 week program with them right before practice began for my Sr. season...I was in incredible shape (I was already on a high intensity weight lifting regiment along with endurance runs and sprint work), the weighted vest mat routines took it to another level. I made starting CB for my Sr. season...good luck...btw not sure how much you weigh, but 40 lbs is pretty heavy. I am 5'11" and weighed about 175 in HS and I was started on a 15 lb vest and worked my way up to a 45 lb vest....good luck