It is quite important that you get access to the environment, even read only privileges if you are to progress with this. It is quite difficult to make recommendation without insight into the environment. If you have a dedicated vCenter for your setup, then this request should not be problem if you go about it the right. But if you are sharing vCenter with other customers, sometimes this is a common practise in hosted environment, then I can see why the vendor can be little reluctant. Also consider getting other IT departments involved to resolve the issues.