Do you plan on using these for visual or photography? These scopes would likely make for a good widefield / beginners photography scope but you might be better off with something else for visual.
I've only ever used the Skywatcher here, so I can't compare them on any more than specs. One thing to note is that if you plan on using the Skywatcher for photography, you will need to buy a new focuser. The stock one is incapable of handling any reasonable amount of weight and it's incredibly frustrating to try and use this. Mine actually broke fairly quickly, and I replaced it with a Moonlite focuser, worth about $500 shipped... and it was worth every cent. I'd expect a similar story with the other two options you've provided, but people who've actually owned them would obviously have a better idea :-)
If you don't plan on using it for visual use, you can get the SW ED BD 80 for about $750 without the eyepieces, diagonal or case. You could use the savings to buy a better focuser.
The Orion is a carbon fibre triplet that's a 480mm focal length and has a focal ratio of F/5.
The Skywatcher is a steel tube doublet that's 600mm in focal length and has a focal ratio of F/7.5. Same for the Vixen.
Bottom line is that the Vixen and Skywatcher have significantly greater focal length, which basically translates to magnification, which is sometimes good and sometimes bad depending on the size of the object you're interested in. The Orion on the other hand has about twice as much light coming through it due to the focal ratio (F/5 = more light than F/7.5). This can be very handy if you're trying to do widefield photography of dim nebula.
Here's some more info on refractors for you, which might prove helpful in making your decision:
http://starizona.com/acb/basics/equi...efractors.aspx
I hope that's of some help... I'm sure someone else more knowledgable will be along shortly :-)