What is the effect of giving a vasodilator to a person suffering from heart attack or stroke?

Can be good: So long as the vasodilator isn't so potent as to cause a precipitous drop in blood pressure with consequent underperfusion of vital vascular beds, it's good. Example: intravenous (so it can be carefully controlled) Nitroglycerin is helpful for an acute mi (heart attack) and often used. The worst thing to do for a stroke, though, is to cause hypotension which can extend the damage.

Think this way: Like stepping on a garden hose cuts down the amount of water coming out of the hose, a vasodilator would remove your foot from the hose allowing more water to flow. Similarly, vasodilators enhance blood flow through a constricted vessel and help blood flow to an area being robbed of that important blood.