But, that's sounding like a good ruling there now: the card playing a Duration card extra times stays in play until the Duration card is discarded (or otherwise leaves play).
Right, that sounds like a good call, because the TR+Duration rule has been that the TR stays as long as the Duration does; the Duration doesn't have to be discarded. That's why Procession + Duration meant that Procession was discarded.
I'm not sure how this is different from my approach. Currently I have a rule, that if you shuffle a card into a deck, it's no longer "that card." I think that's the only situation where I can say that. If the rule applied to trashing, I would then need to explain in the rule how Thief was different; it seems better to not do that.
Well, I assume none of the rulings in this thread, starting with the one you posted at first, are ever going to make it into a published rulebook. But if you mean explaining in forums like this, I think it really makes sense to say that Thief is tracking the card and can pull out that exact card, while Graverobber cannot pull out any specific card. I'm actually trying to follow the rules and mechanics of the game when I reach that conclusion. Before Ingix pointed it out, I thought that Thief and Graverobber work the same, but now I really don't.
Edit: Think about it in terms of a computer implementation where every card is tracked by the game engine. Thief would always gain the card it trashed. When you play Graverobber, either you get to choose a FV in the trash, or the game just chooses one of them when you click "FV". In either case, it would be arbitrary whether you get the one that was trashed earlier in the turn. Of course you can say that this means if you
do get that one, the Vassal stays in play, because the engine knows it's that one. If you interpret it that way, it means that this interaction can only work online, never in a physical game. It's akin to the scenarios where we end up playing a card and not knowing what it is (which contributed to your decision to errata BoM). But another, also valid, interpretation, which doesn't cause the game to break, is that the fact that you can't know which one it is is enough to say that it can't be "that card". Another interpretation is to say that Vassal stays in play if there is one FV in trash, and not if there is more than one. But I think it's better to interpret it as, it can't be that card if you
potentially can't know.