I'm not trying to insinuate anything bad about Dean, and I know Dean got victimized by a vampire too, but do you think there's anything to Dean being associated with Shapeshifters in major ways? Like, in season 1 obviously, but the Alpha Shapeshifter in season 6 ultimately becomes and rests with Dean's appearance, and that seems meaningful.
Oh, tricky! Dean and shifters...
I can't say I've ever thought about it too deeply, but now that you mention it, Buddy the Shifter also takes Dean's visage in 13x04The Big Empty in a majorly visible way, too.
I haven't tracked how the monsters map onto the boys per se, and you're right that often they map to both (Alpha Shapeshifter took on Samuel and Sam too), but I think if I had to come up with something to say about it...
...I'd say it's a fear of Dean's.
Kinda like how "being a vampire" is a very specific fear of Sam's. (-> SEE: SAM'S SUBCONSCIOUS FAKE!DEAN from 4x22: "Listen to me, you bloodsucking freak. Dad always said I'd either have to save you or kill you. Well, I'm giving you fair warning. I'm done trying to save you. You're a monster, Sam -- a vampire. You're not you anymore. And there's no going back.")
That's symbolic in addition to his s4 blood-drinking btw. Sam subconsciously feels and fears being at the kids' table. The burden table. And Sam tries so so hard NOT to be a vampire/burden. He overreaches for power quite often and winds up needing more help than when he started. Augh, Sammy.
But anyway, Dean's fears.
Like, okay. Hang with me. They're solid, well-grounded fears, I think.
In the Alpha Shifter episode 6x02 (I JUST watched it), we get this nifty little scene of Lisa telling Dean, "You don't want to be here." And he emphatically corrects her about that. (He wants to be in both places!) But still, in his life he's hit with a lot of assumptions about what he wants. And like, it's hard to put into words, but I think that hits at a SPECIFIC worry of Dean's, which is that he's being untrue or deceptive. Even when he's trying not to be.
He's used to putting others before himself, and I think it's certainly possible that the he tries to embody what other people want sometimes, too! That could get... well. If not confusing, at least anxiety-provoking.
Hunting is a rough life, and we see that Dean tries to be honest with people he cares about (SEE: Cassie Robinson in season 1), but the nature of his job is... well. Deceptive. That's gotta be a sore spot for him in like this very mind-fuckery kinda way.
And he wears a lot of chameleon-like hats in order to (a) stay safe (b) keep peace (c) keep his privacy. We talk a lot about how the audience sees Dean in specifically uncharitable* ways because they seem willing to buy stereotypes and unreliable narration about him, which... again isn't really born out by his actions so much as preconceived notions about the surface-level disguises he wears, often to protect the things he cares about.
There is a whole intro dedicated to that from the DEAN POV in The WInchesters.
DEAN NARRATION: Hunting's a dishonest business. You gotta lie all the time about who you are and what you do. But the hardest lies aren't the ones you tell other people. They're the ones you tell yourself.
I think the doppel!Dean in 3x10 fits into that anxiety rather nicely too, with him feeling like an Empty thing with nothing he truly Owns or Wants or Needs, maybe even (just) a bad, failed copy of his dad.
DREAM DEAN: No? What are the things that you want? What are the things that you dream? I mean, your car? That's Dad's. Your favorite leather jacket? Dad's. Your music? Dad's. Do you even have an original thought?
Dean fears being empty. He fears "not being a real person."
FAMINE: ย I disagree. (Famine moves closer to Dean and touches him) Yes. I see. That's one deep, dark nothing you got there, Dean. Can't fill it, can you? Not with food or drink. Not even with sex. [...] Oh, you can smirk and joke and lie to your brother, lie to yourself, but not to me! I can see inside you, Dean. I can see how broken you are, how defeated. You can't win, and you know it. But you just keep fighting. Just... keep going through the motions. You're not hungry, Dean, because inside, you're already...dead.
This also throw into sharp relief the moment he actually expresses a need.