Hi Manuel -- Unfortunately, the Baader Zoom will not work in the Pentax scope even with your solution. I took out the compression ring and I even screwed off the barrel of the zoom and it only increased the focus range out to 150 feet or so. It looks like it easily needs 12mm+ of extra travel.
On a side note, the price is most definitely NOT higher for the Vixen. The Baader lists for $189, exactly the same as the Vixen zoom, but the difference is that the Vixen zoom is available for much cheaper because it's been out for a while. Also, Vixen NA was just having a sale where the zoom was $128, and other stores are matching that.
For example, here's the Vixen zoom available brand-new for $129:
http://www.shop.com/op/~Vixen_8mm_2..._of_View_-prod-38314606-50879730?sourceid=298
There's no way a used Vixen zoom should have a higher price than a brand-new Baader zoom.