I def agree that Alastor isn't as feminine as he's portrayed in fanon and Vox isn't strictly masculine, but why do you think Alastor is more masculine than Vox? I kinda thought they were about the same in gender presentation tbh
Part of it is mannerisms, part of it is the way they socialize. Hardly any of it is physical appearance, apart from the way they choose to dress, because body type doesn't affect my perception of gender much (your mileage may vary on that).
Alastor does have feminine mannerisms. Most notably, he often holds his hands in feminine ways, and makes ample use of the "limp wrist" (which makes it little wonder Vox felt comfortable doing the same when they were friends, since it was openly safe to do so).
The two of them also share a few feminine mannerisms that I didn't list for Vox, such as a habit of placing their fingertips on their chests while talking, and standing with their legs together (the latter of which I'd thought was just the way characters typically stand, but in the process of collecting screenshots, I've realized that it's mostly just Alastor and Vox. Other male characters do it, but not nearly as often. So that's an additional feminine-leaning trait for both of them).
We also see that Alastor cares about his appearance (more so than other male characters are indicated to), thanks to his trip to the tailor and a couple instances of adjusting his suit (and he is willing to wear pink, though that wasn't set in stone as a "feminine" color when he was alive. It's also unclear whether he would choose to wear it on his own terms, since all of his preparations in that scene were geared towards appealing to Rosie).
Messing with one's clothes is considered a more feminine trait, and one Vox definitely engages in more. But I actually didn't count it in his case because it's part of a pattern of neurodivergent/sensory seeking behavior, so it's probably not about gender presentation for him (though it could absolutely look that way to people who don't know him).
Posture and seating habits are one of the biggest differences between them in terms of body language; Alastor almost exclusively stands with his hands behind his back or on his staff. When he does lean on things, he generally stays fairly stiff, without the angled hips and loose posture Vox sometimes adopts (granted, part of this could be that Alastor never lets himself relax fully).
As for seating, while Alastor is seen crossing his legs from time to time, he's far more inclined towards propping one leg on the opposite knee, or (mostly in S2) "manspreading".
His time in the spinny chair is also mostly spent with his knees apart, and a number of the exceptions are for the purpose of emoting (or just spinning around). So his overall preference seems to trend towards "masculine" seating (for the sake of completeness, Vox's own preferred position is to put his feet up on whatever is in reach, often with ankles crossed. Which is a completely nongendered pose, though crossed ankles would be considered feminine with a more typical stance).
Transitioning towards social behaviors, it's generally considered more masculine not to show your feelings. Alastor is very emotionally restrained, and under most circumstances, shows positive feelings most easily (to what extent this is genuine depends on the situation, since he does hide his real feelings behind a smile. Though it's worth noting that women are often socialized to hide negative feelings that way). Beyond that, he rarely shows more than irritation, save in his interactions with Lucifer, Rosie (when alone with her), and Vox. Also Husk that one time.
Vox, by contrast, almost never hides his emotions. He does conceal his frustration when talking to the press and dealing with Valentino's tantrum in S1, which is part public relations and part de-escalation tactic (keeping his own feelings from making things worse), and being a manipulator, he often fakes emotions. But typically, unless it actively interferes with his goals to show his real feelings, he expresses himself pretty freely (sometimes more so than he might like, as when he's on the edge of tears after Alastor denies their friendship).
There is also the heart of their emotional conflict in S2, which is that Vox values teamwork and negotiation (stereotypically feminine), while Alastor values self-reliance and strength (stereotypically masculine). Obviously we don't know the full story of their falling out yet, but their values as expressed in S1 also line up with that conflict (Vox trying to restore the peace with the other Vees, Alastor getting upset at the idea of dying for others). So their general worldviews do create a gendered divide in their behavior towards others.
Overall, they are fairly similar in mannerisms. But Alastor definitely carries himself in a somewhat more masculine manner and expresses his feminine traits mostly with his hands, while Vox expresses himself (both in terms of emotions and gender presentation) with his entire body. And their social behaviors are very much split along gendered norms.
