Heroes III: Elixir of Life: A Thief in the Night. Possibility to acquire Vokial

[Alfagar]

Lel, dunno if this was intended to happen by the campaign creators, but as you can see, I got Vokial himself. Looks like if you're quick enough to take a necropolis and wait until a certain map trigger triggers (I suppose it's a time trigger for necropolis owners, allowing the purchasability of Vokial). Regarding that trigger is during the enemy turn, you might need to take all three necropoles to block the enemy's possibility to hire Vokial. Noticed that he's up for hiring accidentally when an enemy hero was going to claim back the most left necropolis.
Whole campaign was played on hard difficulty.

[Amehmet]

Huh, that's an amusing occurrence. It probably wasn't exactly intended, but it's not game-breaking anyway.

Sorry to spoil your fun theorising, but the whole thing is actually much simpler than you figure.

First of all, the selection of heroes for hire is identical in every town you own and in every tavern on the map you visit -- in other words, it depends on the player, not on the particular town. Taking the Necropoles thus plays no role in this case. Technically, I guess the enemy does have a very slightly greater likelihood of getting Vokial at the tavern, but only because their "alignment" (native town type) is probably Necropolis, which means they're guaranteed to get a Necropolis hero on the left of the tavern screen at the beginning of a week. Player alignment is fixed at the start of a scenario and can't be changed during game, but as far as I know, it doesn't affect anything other than possible starting hero (if the player has a "random hero" placed on the map), week's first recruitable hero on the left and the cover puzzle of the grail map.
Furthermore, once a hero is defeated in combat, he or she is simply returned to the pool of available heroes. To the best of my knowledge, they're just as likely to appear at taverns as any hero who has never been in the service of a player -- no time triggers or anything like that. Note that it is possible to make a hero hireable only by certain players or by a certain player, but I guess the developers just didn't bother doing that on this map.

So, to sum it up, what happened to you was pure chance. You can always get heroes you've defeated at your taverns if the map creators didn't make those specific heroes unavailable to you.