Unfortunately (and somehow also fortunate) not in an ideal easy for learning unity/borderline basic users.
The best you can get is the .file/vrca file and run it through AssetStudio (unity asset viewer that can export particular unity game things) or use something like SARS. theres a brief guide already out on some of the current/old/etc if you keyword search around on here. though I only recommend lookin into it if you've got the patience for the steps needed and any hiccups that can roll around like saves being bricked or corrupted from a misc script oversaving.
note you will have to have general knowledge on unity and blender if you plan to edit things and it in chance all breaking on import. this video is one that I've passed over and was helpful in everything I needed to figure out stuff with on recovering some of my old lost project models.
Hope this helps in some way and best of luck!
Yea check all of your facial animations, record the ones missing it may be forcing the blendshapes you're using to zero hence returning the face back to default
@kittyyyy ofc, also to add: take a look at d4rk's optimizer for a really well made tool that uses custom material trickery among a lot of other methods to optimize even the heaviest avatars
is it rigged? if so it's a rather straightforward process, though you might need to make some changes to the armature to work with the expected base humanoid rig. you can make visemes using the CATs blender plugin, i think.
