Some of the neos on this forum often cite Tulasi Das as evidence of the sameness of Shiva and Vishnu. But aside from the fact that Tulasi Das is not shruti, I have a friend initiated in the Ramanandi sampradaya who tells me that their view (including the view of Tulasi Das), is that Vishnu only is supreme, and that Shiva, though a highly-regarded deva, is not the same as Vishnu. What little I have read from Tulasi das seems to support that view.

I agree fully that the doctrine of "all gods = the same god" is not Vedic by any stretch of the imagination. That being said, doesn't Adi Shankara equate Shiva and Vishnu with his pancopasana doctrine? I have heard it said that pancopasana might have been a later development, but am not aware of any statements by Adi Shankara one way or another on this specific issue.