If Outpost Siege (choosing Dragons) leaves the battlefield at the same time as any of your creatures (e.g., from Perilous Vault), the game looks back to see that it should trigger, and you'll get the 1 damage per creature you were hoping for!
Tag: Outpost Siege
Outpost Siege and Dash
Outpost Siege lets you cast the exiled card, but it doesn't mandate any kind of alternative cost (such as "without paying its mana cost"). That means you can choose to pay the Dash cost for the exiled card, and it will return to your hand normally at end of turn.