I just went through this exact same dilemma, with $600 in my PayPal account dedicated to binoculars. Originally, my mind was set on getting the Vipers, and hadn't even heard of Zen Ray. I bought a Vortex riflescope last fall that is incredible. Then I started reading into the Viper even further, and saw some complaints about a narrow field of view, and some people described it as looking through a tunnel. That got me considering some other options.
In reviews the Zen Ray ED3 seemed to hold its own against the Viper, and even win out in many of them. The ED3 runs around $400. Then I started looking at Prime comparisons to the ED3 and many of the users said it was an incredible jump up. Boom! Mind made up.
I was blown away at the difference between my old Nikon Monarchs. Honestly, they worked very well for me, and I was hoping I wouldn't even like the Zen Rays so I could send them back. After a two hour comparison, there was no comparison.
There is one caveat that has me a bit concerned, but will likely never be an issue. 99.9% of customer service reviews on Vortex are incredible. I don't know that I've found a bad one. If you look, you can find some about Zen Ray. Not horrible, but things like unreturned phone calls and such. In that respect, I feel like I took a bit of a gamble, but it will very likely not matter.
Best of luck. Hopefully I didn't offend an Vortex guys, as I'm sure those are great as well.