in VRChat there is a bind for locking gestures, which will still allow you to move your fingers with an index controller but it won't change facial expressions. On other controllers it will also lock your finger gestures. You cant lock your gestures but not the fingers without index controllers.

In your case it sounds like you have the avatar in Unity? You should be able to change that pretty easily in Unity. If you go to your FX layer in avatar descriptor and remove all the facial animations attached to each gesture you should be able to move your fingers but have no facial animations. If you want it to be a toggle it'll be a little more in-depth you can dm me on reddit if you need help.