I'm not sure what your expectations are for "starting and stopping a custom ViewModel manually" -- but a ViewModel should always be scoped to the ViewModelStore and should only be cleared when the ViewModelStore is cleared. You shouldn't be trying to manually control that; and if Koin allows that I would really recommend against.
If you want control over the lifetime of the ViewModel instance, you probably don't need ViewModels. Create a custom scope, a custom class and handle the lifetime of that class instance yourself.