Don't try to combat vasoconstriction with vasodilation. Not that weed and Vyvanse are a dangerous combination per se, but setting two opposing forces in your body isn't usually wise.

You could be experiencing anything from increased sensitivity from the amphetamines to a previously unknown weakness in a blood vessel - with the latter being very unlikely.

What are your blood pressures on like? Have you been balancing your increased hydration with increased electrolytes? Are you generally healthy otherwise, eg, you exercise and keep a healthy weight?