I read a lot of Pietro Maximoff fics, and the emotional vulnerability thing feels super consistent across writers. They almost always make his speed a double-edged sword—it's not just a power, but a source of isolation. The fics I gravitate towards explore how being that fast means the world is slow and blurry for him, making genuine connection hard.
Because of that, the moments where he slows down for the reader character feel massive. It's rarely a grand confession. More often, it's him letting his guard down in a quiet moment, maybe exhausted after overusing his powers or just sitting still, which is a huge act of trust. The vulnerability reads as physical stillness paired with emotional exposure, which is a neat metaphor.
Some stories lean into his canon abrasiveness, having him use sarcasm or impatience to cover up when he's feeling too much, which can be done really well. You see him lash out and then immediately regret it, struggling to articulate why he's upset. That feels very true to a young man who's used to running from his problems, literally and figuratively.
It's a specific flavor of 'show, don't tell' hurt/comfort, I think.