Been observing this thread with interest, although couldn't really add much more to what's already been mentioned. Certainly a great looking vase, and great set of new pictures, which show the detail well.
As already mentioned, the base and rim are concerns for concentrating on IOW, I would tend to lean towards either the piece being a piece of studio glass or Alum glass which has been aforementioned.
I had a look at a few examples of Alum Glass on the web and in terms of the base finish and treatment, Alum does seem the closest at present to yours. I would be very interested to hear what Alum think. Also, as mentioned, whilst contacting IOW, may well draw a blank, they may well be able to suggest another avenue to explore.....
While looking across the web, I came across a recent thread on GMB regarding Alum, whilst the piece in the thread has clear differences, there may be some similarities to yours....(not sure if you have seen this already..?)http://www.glassmessages.com/index.php?topic=40900.0
Interesting to hear what you find out, certainly keep hold of the vase for a little longer yet!