I believe there's two different equations you want to look at there. The principle you want to remember is that the body is always trying to keep flow constant (steady state). In general, you can think of this in terms of the formula Q= AV and Q = P/R. Let's consider what happens to VELOCITY first. First and foremost, vasoconstriction would result in a decrease in cross-sectional area, resulting in an increase in blood velocity. Vasodilation would result in a increase in cross sectional area, and decreased velocity. Again, when the body is in STEADY STATE, it tries to keep flow constant. Now lets look at PRESSURE. In regards to pressure, we want to look at Q = P/R. By vasoconstricting you are essentially increasing the resistance to flow. In order to keep flow constant, you would see an increase in pressure. The opposite goes for vasodilation. Again, the body always wants to keep flow CONSTANT. If it doesn't, then there's something pathological at play.