I noticed that my vampire follower (a custom follower mod, who was not a vampire) has his face mesh temporarily gone when I enter an interior/exterior cell and he doesn't go with me, or 'forced' teleport. His face mesh goes back when I open Companion Share, then re-equip anything he wears. So in general, the vampire face mesh disappears for conditions like that. Is there a way to fix this?

Edit: I forgot to say that this problem gets fixed by itself sometimes.

probably yes, but it must be scripted e.g. to add, equip, remove a helmet to the npc on cellchanged. Note that using Melian's mod for teleporting should have the fix included already

Something funny I noticed is the opposite: while you make player talk using the say command (e.g. when daedric shrines talk to you), player face is vampiric

I don't need a mod that makes followers teleport to me, it's just the vampire face mesh disappears from time to time, that's the issue.

I just explained how it can be fixed using a script and gave you an example of mod having the fix already implemented in case you want to script it yourself, or use the mod to teleport the companion and try and avoid the problem. I don't know a specific patch already done for vampire embrace, it is an old mod and I suspect the problem was not known when it's been done else surely Cortex wouldhave fixed it.