Why is it that you need to dilate the veins to avoid tachycardia induced by drugs?

Not clear: Your question is not clear. Please reframe it. We don't dilate veins to reduce tachycardia.

Cart before th horse: Purely vasodilating drugs can induce a reflex tachycardia. Examples are hyralazine and nifedipine. It is for this reason that these drugs are not used by themselves in patients with angina but must be given with a rate modifying drug, often a beta blocker. And it's not venous dilitation, it's arterial dilitation.