|MadSci Network: Anatomy|
For the first part of your question, there are a number of chemicals which act as vasodilators. There are, in fact, too many to list all of them, but just a few are capsacin, nitrates and theobromine(found in chilis, preserved meats, and chocolate, respectively). Inside the body, histamines, adrenaline, and heparin also increase vasodilation.
Increased pulse rate can occur from anything that increases the oxygen and nutrition requirements of the body, increases body temperature or which triggers the release of vasodilator or stimulant chemicals. Physical activity, fevers, being out on a hot day, being frightened or excited all can cause elevated heart activity.
You are correct that some people are relatively insensible of their heartbeat or pulse and there are some who are hyper-aware. It's even possible for some people to have a condition called atrial fibrilation - where the atria of the heart contract at enormously increased rates - and have no sensation of it. I haven't come across any solid explanation of why this disparity of awareness exists.
I hope this is helpful!
Try the links in the MadSci Library for more information on Anatomy.