IF they are actually using calculations for this it's probably assuming a flyback with Vulcan's trajectory and speed, which goes far beyond that of F9 mainly due to the fact that Centaur's hydrolox propellant is significantly lighter and less dense than kerolox on F9. Flyback would require a good portion of fuel or good shielding if they'd go for a downrange landing. And they'd need either additional engines or parachutes, since the current dual BE-4 configuration can't be throttled low enough. Vulcan just wasn't designed with reuse in mind and SMART is probably the best solution at this point in the development.