Re: Vortex Viper HD 10x42 Binocular
Damn, it sounds like you need 10x or more for ranges like that. Nothing wrong with that, it's just more binocular than what I use.
Again, I'd still suggest that you keep the objective diameter at 50mm or above. Even though it is a very common configuration, the 10x42mm binocular makes for a tight eye-box, and dark images in lower light.
What I ended up doing was buying several pairs so I could make a side-by-side comparison. I found that
Eagle Optics worked out quite well for this. After talking to them, they actually encouraged me to do it. They have an excellent return policy. Thirty days, no questions asked. And they are super prompt on refunds. I ordered around $4,000 in binoculars one week, and returned all the ones I didn't like. They had my money refunded to my credit card by the next week. It worked out very well, and having all the binoculars available to me at the same time was priceless
As you may know, Eagle Optics is essentially a store-front for Vortex, so they certainly pimp that brand a lot. I ordered a pair of Vortex Viper HDs along with my other selections. In the end, I didn't find that the Vortex's performance was as good as some others. Sure, they have a VIP warranty, but it seemed to me that optical quality suffered because of it. In other words, I felt like I was paying more for a great warranty than for the optical performance. I know that isn't the opinion you hear a lot about Vortex, so your mileage may vary.
Nonetheless, for your application, I'd recommend you take a look at these options;
Vortex Viper HD 10x50mm
Vortex Vulture HD 10x56mm
Leupold Mojave 10x50mm
Pentax DCF 10x50mm
Fujinon Polaris 10x50mm
Minox BL 10x52mm
Celestron Granite 10x50mm 
