Played my first game last nite with a friend, a learning game. He was Imperial and did a "what the hell" boarding action against the Vengeful Spirit. He did well, boarded more troops and suddenly poor Horus was trapped and eliminated. Upon a quick look, I couldn't find any way for the Traitor to bring troops back up to the Vengeful Spirit, nor could I find a way for Horus to bug out (the Imperials held the Catecombs). Are either of these possible?

Port Landing when executed in the VS region allows the Traitor to move troops from a spaceport to either area of the VS. 6 ranks if ex from hand, 12 if from strat map. But realistically, if the Emperor boards and Horus through some utter miracle (since Emperor should be bringing minimum 3x Custodes, 3-6x Space Marines with Dorn and Sanguinius with him) is still alive, he needs to come down to Terra, ASAP.

For bugging out, Port Landing or Drop Pods both allow the Traitor to move troops from VS to Terra, Horus can piggy-back with them.

Also, a port landing played in the vengeful spirit region will allow you to bring units any units on a spaceport into either vengeful spirit area. It says so on the game board itself just under the vengeful spirit.

As said above, it is pretty easy to move Horus from the vengeful spirit to terra. Not that port landings and pod orders allow you to choose units and heroes from the stockpile as well as the vengeful spirit. A specific vengeful spirit area is not specified so assume that you can do it from both. After moving troops into the vengeful spirit, if the Imperial player does not kill horus before a change of current player then Horus will just bug out.

 Ah, I see ... I would have actually had to have read the rules thoroughly to have the answer to my questions! :)

I do now see the description of 'Port Landing' in the Vengeful Spirit section of the rules, and more carefully read the Drop Pod rules. 

Horus could have fled, er I mean tactically withdrawn to the surface to personally lead the seizing of the spaceports.

Thanks for the pointer.