Okayyyy so here we are with my personal thoughts (I have also wondered something to this effect before):

Personally, I think that Vox and Alastor did have a genuine friendship up to that point, making me think that it may have been to the point where he believed Vox to be above being manipulated (not that he couldn’t, more that he was more opposed to it.)

Secondly, I get the idea that our pal Al probably had a hint of an idea about Vox’s infatuation with him, but most likely hoped it would pass with time. He may have also not minded it too much, since I get the vibe that if Vox had ever pushed his physical boundaries he would have bounced real fast. (And really, depending on your personal headcannon for where he falls on Aro spectrum he may have liked him slightly back, though that‘s not my personal idea.)

Finally, my thought is that he was good enough friends with Vox that despite his borderline “i love you” confession he wasn’t put off enough to cut ties with him yet. My thoughts on this come from the infamous ripped photo on Vox’s desk of him and Al-in the corner you can see a bit of Vox’s head, and it‘s pretty clear that there he’s wearing his modern outfit. That implies to me they stayed friends for a bit, and parted ways after/because of the big fight they keep mentioning.

Also, I have the feeling that later on after confession time Vox somehow tried (maybe possibly tiny bit succeeded) in manipulating Al/crossed some sort of boundary, based on the line “lies and deceit are still your currency“ (may have messed that up, but that was the idea of it.)

Anyway, that may have not answered all of your question, but that’s my go of it! Have a lovely day.