Vasodilation changes blood flow and this could easily influence something like a varicose vein. Many people with a varicocele report that (for example) pycnogenol causes pain due to altered blood flow.
As somebody who is always willing to experiment, I listen to my body and if a plant/substance turns out to be less than an "ally", I reject it for personal use. Whether that plant would be more useful as a formula is subject to knowledge of synergism/formulations.
It would be nice if plants didn't have side effects, which almost totally undermines herbalism for me (subject to more enlightenment from someone).