Being a physio, I would say It sounds exactly like tendonitis of the extensor tendons of the foot, most specifically and commonly of the big toe. Totally adds up with the symptoms your describing, and running in VFF would certainly make this worse. The extensor tendons of the toes are meant to work as a group not individually and VFF's make you isolate each. Firstly have a couple of days off. Do some gentle long stretches, by pointing toes down then applying pressure, such as by leaning down on front of toe nails in standing (1min minimum 3 x daily). Warm the front of the leg from mid shin up with a hot water bottle and massage the muscle at the front of your leg (the one you can feel move as you wiggle your big toe). Also take some antiinflammatories (neurofen). I had exactly what you descirbe 1 week ago, did all of the above and its totally gone. Hope this helps! Oh and when you resume running, go easy on hills for a week or so, as it makes you extend your toes more than normal running.
Jules