Darksteel Citadel's (hereafter, DC) crystallize ability is supposed to trigger whenever a colorless card is cast, and crystallize is supposed to give you the option of exiling any support in play, including vanguard supports. And it doesn't work right. Let me illustrate the bug.
Say you already have a DC in play and there is also a Vanguard support in play. If you cast a colorless card that is something besides an additional copy of DC, then you will have the option to convert the Vanguard support gem to a crystallized gem and to exile the Vanguard support. In other words, DC works as it should when you cast a colorless card that is something besides an additional copy of DC.
But if you cast a second copy of DC -- which is itself a colorless card -- the crystalize menu will trigger (by crystalize menu, I mean the option to select among the various supports on the board and crystallize one of them), but you will not have the option to select the support gem that has the Vanguard support on it. You will instead be limited to selecting gems that have supports that are not vanguards. So DC is not working right when the colorless card that is cast is another copy of DC.
Here are some screenshots showing that I cast of fully charged dark steel citadel, and that it allowed me to crystallize many different supports, but not the Vanguard support in the lower left corner of the screen. I already had a dark steel citadel in play when I cast the fully charged one in my hand.
Image 1
Image 2
Image 3
Image 4. Note that the game is not giving me the option to crystallize the Teferi vaguard support gem in the lower left of the screen (the Vanguard support gem is just above the corner gem in the lower left-hand portion of the screen).
. Note that the game is not giving me the option to crystallize.
I have always wondered why that happened, kudos for figuring out the reason
This was SOOO specific.. but true! We're taking a look at it!
Lu