Sadly, this doesnt seem to work when in fallback mode. There is no error, it simply doesnt work. Tested in Chrome (useProxy: false) and internet explorer.

When using the proxy, it works like a charm (only supported browsers of course)!

<input v-model="formData['key.value']" />

computed: {
   formData () {
        let model = this.$deepModel('key', { useProxy: false })
   }
}