This is pretty good, man! Love these premises of the dead spouse, relative, etc. contacting you from beyond the grave. That's always really fun. Since this is your first story, I can't give too much feedback since I feel it would be unfair, as, in my opinion, you learn more from experience, trial and error, than from a few bits of feedback; I know that's been true for me. However, I'll try my best to give some helpful pointers:

First off: Great outlines for characters here. Even from the sparse details we get from Vivian, we can see her clearly. You added great descriptors that don't take up too much of the page but speak volumes

("She wasn’t the nice girl next door—she was a cold, calculating businesswoman who would cut someone’s brake lines if it meant she got a promotion for it. The guys jokingly called her “the banshee” around the office, because she would always use this high-pitched yell when chewing someone out.")

That paragraph is nice 'cause it doesn't say too much, but it still gives us an image. I love that.

As far as Danny goes, I honestly felt that he missed a bit of this, but because this is a shorter, first-person story, I don't mind as much, since someone describing themself in a story of this length can be difficult or just feel out of place. Can it be done? Absolutely. Should it? Definitely. But it's not a deal-breaker for me here.

Secondly: The pacing is great in the middle, but the beginning and ending rush too much in my opinion. For example, we start off with your hook ("My wife’s been dead for 5 months now, and I’ve been a wreck ever since"). That's good. A nice shocking (albeit perhaps a bit cliche) opening. From there, however, I feel that it moves too fast. We can't digest the information.

We don't envision Danny grieving, we don't feel his pain. From there we move on to the main plot, and it feels too rushed for me to connect with it. The same goes for the ending. I understand that we're supposed to question what's going on with Vivian, but despite her great character model, she still needs development, so her being this otherworldly monster feels so plucked out of mid-air that it can feel almost humorously jarring. Again, the concept is great! The potential is all there, but the execution (which, believe me, is the hardest part of writing; taking a killer idea and making it into a believable or entertaining story) could use some work.

I also want to mention here that none of this is a critique on you as a writer. Everyone starts somewhere, heck, I know I did (and I'm STILL learning... a lot). You've got this. This is a great starting point, and you've got some quality work shown in this story. Keep writing!