Sure. I just think it is a little bit of moving the goalposts (in either direction) to change a creatures resistance/vulnerability based on an ability check.Yeah, my scenario with the monster ability is a little bit out there, at least for D&D. But I wonder how much mechanical impact it would actually have. Sure, "remembering" that this monster is vulnerable to your primary attack spell would be a huge bonus, but what if you fail and suddenly it's resistant or immune to it? So it's really a matter of where the DC lies.
Still, it's just not very D&D. I get that.
However, I think it would work well in the "closing the portal" scenario, provided the DM hadn't made closing the portal by some other specific means an integral part of the adventure. In other words, if the players are going off-script (as it were) just by trying to close it, the solution of making the Arcana check as you try to close it could be fun and memorable.
(And maybe even in some constrained or even unique circumstances it would work with monster abilities; I just wouldn't want it to be a regular thing.)
I'd have to think about the player who asks: "What is this creature's vulnerabilities?" I'm kind of on the fence on this... I kind of think if I can't think of what would happen if they fail the roll, why would I allow a roll to begin with. My assumption is that the skill check to make this identification is an action used in combat. The player forgoes an opportunity to act for the chance to succeed on an information recall.
This falls in line with my trinary approach...
1. take the risk and succeed gain knowledge on the creatures weakness,
2. take the risk and fail gain no knowledge and lose your turn,
3. don't take the risk and take your action.