If I play Void Rush and one of the two cards I reveal is Vanguard Captain, do I get the legion effect if I didn't play anything before Void Rush?

Ruling

Yes, you do get the Legion effect.

When you play Void Rush as the first card of the turn and use it to play Vanguard Captain, the Vanguard Captain successfully triggers its Legion effect.

Sequence of Events

1. Play Void Rush: You pay the costs and place Void Rush on the chain. It is the first card played this turn.
2. Resolve Void Rush: You reveal the top 2 cards, choose Vanguard Captain, and play it.
3. Vanguard Captain enters the board: The Vanguard Captain enters the board. Its "When you play me" ability is placed on the chain as a pending item.
4. Check Legion Condition: The game checks the Legion condition: "If you have played another Main Deck card before this one already this turn."
5. Legion Succeeds: Because the Vanguard Captain is played during the resolution of Void Rush, it is considered the second card played this turn. Even though the spell is still resolving, the game recognizes that a Main Deck card (Void Rush) was played prior to the Vanguard Captain, satisfying the Legion condition.

Interaction with Spell Resolution

While Rule 154.3 requires finishing the resolution of a spell before addressing triggered effects, the Vanguard Captain is considered the second card played in this sequence. Per the established logic for similar interactions (such as Portal Rescue), playing a card during the resolution of a spell allows that card to count as having been played after the initial spell, thereby fulfilling the requirement for Legion.

Note: I could not fully verify this ruling against the retrieved rules.