A knock on my door. Sleepy as I had almost fallen asleep, I rolled out of bed onto my week weak feet. I dragged myself over to the door.
I've never heard of weak feet in this context. "I rolled out of bed onto my feet" is fine and natural. We often say we're weak in the knees from fear, or "My legs felt like jelly." "Weak feet" just doesn't fly. I don't know. Maybe, "I rolled out of bed onto my feet, but they didn't want to support me. (OR refused to support me.)"
Too tired to look through the peephole, I opened the door a crack with the bolt chain still on, cursing whoever was disturbing myself me at this late hour.
I don't think in the US we'd say "I opened a crack" in this context.
But much to my pleasant surprise, a sexy doll-like young woman, probably in her early twenties, looked at me with a wide smile and big brown eyes. (good)
[NEW PARAGRAPH]
It was a bit chilly. I could feel the cool breeze brushing against my exposed neck, hands and feet, wearing only boxers under my blue house robe.
Logically, the order of the last sentence should be reversed, placing the participial phrase first. As it is, "exposed" makes no sense to the reader.
Two more logical problems: (1) Do you normally sleep in your house robe? You say you were in bed, nearly asleep. You describe getting out of bed and going to the door in minute detail. When did you put on the robe?
(2) Where is the cool breeze coming from? You only opened the door a crack. Are your windows open? Your door opens into a hallway. Why would there be a breeze in the hallway? (The girl is supposed to be effected / not effected by the same cool breeze.)
She stood calmly there, seemingly not shivering, with only her sheer, short red nightie covering her petite body, exposing her vulnerable arms, shoulders and most of her legs.
"Vulnerable" is out of style, as my music composition professor would have said. I know what you mean. You could describe the girl as vulnerable. Somehow "vulnerable arms" seems ugly.
I think the "exposing" phrase would go better as "leaving her etc. etc. exposed."
Though the hallway was dim dimly lit, you could tell she wasn't wearing a bra. My eyes were wide open by now and when I was about to ask her why she rang my doorbell, she put a her hand on my door and gently said, "Can I come in?"
"Her hand" is more personal/tender?
My natural answer was would have been no (OR My instinct was to say no,) but before the word came out of my mouth, my mouth swallowed and replaced it as if it had a brain of its own, "Yes, sure". I shut briefly closed the door, unlocked it and open to unhook the chain, then opened it wide with an inviting gesture, "Come on in".