Some rough thoughts:

A 60 degree vbit is a cone. For each depth, there is a circle of a different radius.
If a maximum depth is selected, There is a circle, maxCircle, of maximum radius.

For two intersecting curves, using the Moi tantan circle, a series of largest circles can be created touching the upper curve,
and also touching the lower curve. If the separation between the two curves exceeds the maxCircle, the maxCircle would hug
the upper curve. The zDepth of the center of the circle is determined by the vBit angle.
The centers of all the tangent circles would create tipCurve, the path of the tip of the vBit.
Then sweep a line along the top curve and the tipCurve.

Repeat for the bottom curve if maxCircle is exceeded, and also form plane between tipCurve1 and tipCurve2.

(This is a rough "algorithm")

- Brian





I did locate a Rhino/Grasshopper reference: http://www.grasshopper3d.com/forum/topics/tangent-circles-between-2-curves