Both versions are acceptable, although the version with the -ing verb form needs a minor tweak:
When I realized that no one was paying any attention to me, I decided to go home. But just as I was leaving the party, a beautiful blond girl walked through the door and smiled at me.
When I realized that no one was paying any attention to me, I decided to go home. But just as I was leaving the party, a beautiful blond girl [was] walking through the door and smiling at me.
The idea that one action interrupts another is by no means a given. What would that mean? She was walking but then had to stop in order to smile? People can do many things at the same time—including walking and smiling.
Assuming that two actions aren't happening at the same, time, one action doesn't have to interrupt another;another one; it's at least as common, if not more so, for one action to simply follow another one.
The question here is, does the woman walk through the door and then smile at you—or do you see her walking through the door as she is smiling at you?
Both versions are grammatical. It's what actually happened that determines which phrasing is the most accurate.
Also, it's a mistake to think that the verb form itself determines if two actions happen sequentially or in parallel:
When I realized that no one was paying any attention to me, I decided to go home. But just as I was leaving the party, a beautiful blond girl walked through the door [as she] smiled at me.
When I realized that no one was paying any attention to me, I decided to go home. But just as I was leaving the party, [I saw] a beautiful blond girl walking through the door [before stopping] and smiling at me.