Thanks again for the report!
First, if you want to upload a picture, click on "Attachments and other options" just below the area where you type your post, then select "Attach".
I *think* what's happening and what is confusing you is the difference when playing the card with Vassal is your only option, vs. when you may be able to do other things.
The case 1) would be hand of say 4 Copper and Vassal. You play Vassal and it hits a Smithy. In this case, you either play that Smithy, or your Action phase is over. That is presented with
a) a "Play Action" and "Don't play" button in the play area,
b) a message in the central information area saying "You may play a Smithy",
c) a messege in the log area saying "You may play a Smithy from your discard pile"
I *think* when you wrote
Usually if Vassal hits an action card it will tell me in a message what the card is
you are referring to my part b) above.
Now the picture changes a bit if you have other options. In case 2) that's usually the case if you have Actions available and Action cards in your hand. In your game #24522571, you had played Festival before (so one Action to spare) and you had Militia in hand.
So when you played Vassal and it hit another Festival, you had 2 options: Play the Festival from your discard pile, or (ignore the Vassal effect, leave the Festival where it is and) play the Milita from your hand.
This is presented in the game as follows
a) a "Play Action" button in the play area,
b) a message in the central information area saying "You may play Actions",
c) two messages in the log area saying "play an Action from your hand." and "You may play a Festival from your discard pile" (they can be seen in what you copied into your post)
d) the "End Actions" button in the char area.
The difference to the previous case:
a) no "Don't play" button. Not playing the Festival is implicitely done by either playing the Militia or pressing "End Actions"
b) the message in the central informations area is showing only one option
c) two options are shown in the log area instead of just one.
The general idea behind this whole setup (that happens in many cases with different options) is that in the log area, text can scroll, so the game can present all options, and often
(but not in this case) requires selecting the option by clicking there.
In the central information area, the space is limited, so only a short guidline can be shown. Unfortunately, in the second case with options, instead of the central information area telling you to choose between the options, it shows incorrect text that makes you believe you cannot play the Festival.
So I can see that if you usually encounter case 1), the incorrect text in the center can mislead you in case 2). OTOH, the button "Play Action" is there, and AFAIK it exists only for Vassal or similar cards (Herald), so this should be a good indicator that it should be clicked to do what you want, just in case 1).
At the core of the confusion lies what has been negatively remarked before: A player should not need to look into the log area for 'normal' game interaction, like Vassal.
I hope this was helpful, if not in solving the main issue (you did that yourself) but in showing the different parts of the game where important information may be shown.