Two things:
Generally when you take medicine with high acidity food, like coffee, it will break down your medicine prematurely - you won’t get as strong as an effect. I had to stop taking my Vyvanse with orange juice because it basically nullified the medicine.
Talk to your doctor. I had chronic headaches that I found out were related to low blood pressure and dehydration. Drinking water instead of diuretics, like coffee, immensely helped me.
The answer to your question is to calculate your BMR, or basal metabolic rate. It’s an approximate calculation based on your sex, age, height, and weight. Your BMR is the amount of calories you need to maintain life when completely sedentary. If you only eat that number of calories, and do ANY physical activity, you will begin to lose weight as your body taps into your reserves for energy.
There is more nuance, but the simple math is calories in vs calories out. If you have a deficit, your weight will decrease. People counting calories to lose weight generally target their BMR minus 200 to 500 calories daily, though 500 is a bit much in my experience.
Talk to your doctor, but the simple answer is to take more. Vyvanse has a half life of about 12 hours, so if you take it in the morning then by evening the medicine is starting to heavily breakdown. It’s pretty common for people to take more at lunchtime to get them through the evening for that reason.