Rated Game #7559582, turn 13.
Played Vassal, and it discarded a Vassal. It did not prompt to allow playing it.
Undid, and played it again, and it still did not allow playing the discarded Vassal.
I loaded your game and it was possible to play the discarded Vassal on turn 13. You have to click the "Play Action" button below Delve. (Note that alternatively you can click Goons in hand to continue with your turn without playing the discarded Vassal, which you probably did.)
Hmm, I was so sure it wouldn't let me, but I didn't take a screenshot, oh well. Thanks for looking into it.