If magical connections were two-way, I'm pretty sure an experienced wizard would know about that and install some kind of protection. Like say... putting some protective wards around the vial in case the other guy tries sending magic through it. I mean, in the real world we've got all sorts of tools and guidelines for handling potentially dangerous, toxic, explosive, or radioactive materials. Do you really think Cursitor Doom is going to take some bits of some guy's soul and use it for magic without doing the wizard equivilent of sticking it in a ziplock bag first? Or put some wards on it to act like the magical equivalent of a lead box, or a surge protector, or a firewall, or whatever else he needs?
Second, even if the connection does go in two-directions that doesn't put both sides of the connection on equal footing. For example, the connection could be like a magical pair of jumper cables hooked to a magical car battery and clipped onto some part of Grayven's body that doesn't want jumper cables hooked up to it. Considering it's his soul that's making the connection... those jumper cables might as well be hooked up to Grayven's left eyeball right now.
"Aha! You may have a dagger pointed directly into my eye, but that just puts your hand within headbutting range! I bet you didn't think of that, did you? Oh, wait... you totally did. Rats, I was kind of expecting being totally at the mercy of someone experienced at putting people at their mercy would give me more of an advantage than it actually does."