It looks like you should be able to access and set default options in Vue.http.options and Vue.http.headers. This appears to be their default values respectively:
{ // options
method: 'get',
params: {},
data: '',
xhr: null,
jsonp: 'callback',
beforeSend: null,
crossOrigin: null,
emulateHTTP: false,
emulateJSON: false
}
var jsonType = {'Content-Type': 'application/json;charset=utf-8'};
{ // headers
put: jsonType,
post: jsonType,
patch: jsonType,
delete: jsonType,
common: {'Accept': 'application/json, text/plain, */*'},
custom: {'X-Requested-With': 'XMLHttpRequest'}
}