I wish I was kidding but this has actually helped me get 2nd vanguards.

At the start of the hero selection/starting room, I type in the chat: "I'm so lonely, nobody wants to play vanguard with me."

Instead of straight up asking someone to play a 2nd vanguard.

This has massively increased my chances of getting a 2nd vanguard.

I'm assuming its because people feel like its more humble and sad, instead of being ordered to play something.

My results were from GM1-GM3 matches.