You should take your multivitamins in the morning with a meal so you can ease absorption. However, if that causes stomach pain, try taking it before bed.
Consider eating a complete meal of carbs, protein, and fat within 2 to 3 hours of exercising, or a smaller meal focusing on carbs and protein 1 to 1.5 hours.
An ideal time to eat a pre-workout meal is about 1-1.5 hours before exercising. If you eat and wait longer before exercising, you run the risk of having low.