  Vega has been shown to host multiple dust populations, including both hot
exo-zodiacal dust at sub-AU radii and a cold debris disk extending beyond 100
AU. We use dynamical simulations to show how Vega's hot dust can be created by
long-range gravitational scattering of planetesimals from its cold outer
regions. Planetesimals are scattered progressively inward by a system of 5-7
planets from 30-60 AU to very close-in. In successful simulations the outermost
planets are typically Neptune-mass. The back-reaction of planetesimal
scattering causes these planets to migrate outward and continually interact
with fresh planetesimals, replenishing the source of scattered bodies. The most
favorable cases for producing Vega's exo-zodi have negative radial mass
gradients, with sub-Saturn- to Jupiter-mass inner planets at 5-10 AU and outer
planets of 2.5 to 20 Earth masses. The mechanism fails if a Jupiter-sized
planet exists beyond ~15 AU because the planet preferentially ejects
planetesimals before they can reach the inner system. Direct-imaging planet
searches can therefore directly test this mechanism.
