Both lynx and bobcat are possible throughout Vermont, though lynx is mostly restricted to the Northeast Kingdom. Your description sounds a lot like a lynx, but...
- estimating size of a rapidly-moving, quickly-glimpsed animal is just not reliable, even for experienced wildlife-watchers. Being off by a factor of two or more wouldn't be surprising at all.
- neither lynx nor bobcat is what I'd call "charcoal grey", at least in the summer. (I've seen a lot of charcoal and none of it was grey, but that's another story.) They're more brownish, with bobcats tending to be a bit more rufous than lynx, but then again, both are pretty variable in color and you don't say whether the light was good.
So pick your theory, in order from mundane to exciting:
- coyote or silver fox or domestic dog (not very catlike, very obvious bushy tail)
- grey house cat (unusually large?)
- fisher or some other large-ish weasel (not grey, long obvious tail, but their shape and movement can be catlike)
- bobcat (not especially grey, kind of unlikely in daylight, but you didn't specify what time of day)
- lynx (not especially grey, quite rare even in northern Vermont)
-puma (cub?) (not especially grey, long obvious tail, no breeding population proven in the state)
