Update:
After looking into this; it seems that it's not really in the design of Vue.js to bind and react to Javascript variables outside the Vue instance sandbox.
No big deal... we can manage without this capability and indeed, as Scott has pointed out to me, there are probably good security reasons for this design (Vue.js as a sandbox).
However, in the interest of being complete, I asked this question over at the Vue forums:
Bind Vue.js variable to external Javascript variable for reactivity - Get Help - Vue Forum