I had the jumpiness problem with my VT, too. When I put my arm in the tank, he likes to bite the freckles on my arm. Of course, it doesn't hurt that much, but the surprise of it always made me jump and freaked out Adorable Fish.
What I did was just try to keep an eye on his location, because I found that if I was expecting it I could control my startled reaction. Now, when I stick my hand in there, I tell myself that I could be bitten at any moment to try and psyche myself up for it. It seems to help.