Top of the list will be the okna followed by the vytex, the berkshire is a distant third. The berkshire cant even be offered in triple pane glass, not that you need it. That tells me the frame isnt that strong.
As an aside, if you search hard enough, you can find something negative about any company. I always say to evaluate the general theme of the company.

I think you know the answer and I agree with you.

Persoanlly I'd toss the Berkshire, and the Thompson Creek has a pocket sill which is not ideal.
That leaves the Vytex and Okna. The Fortis is a pretty good window, but the Okna rates better and would be my choice. The price is reasonable and the dealer sounds like a slam dunk. Easy choice IMO.

Thanks for the feedback everyone. I took another look at the Vytex windows and was satisfied that they are a quality window. I also liked the installation company, whose rep was helpful and not at all pushy. Most of the negative feedback about that company related to rude salespeople, but that wasn't my experience at all.

Although the Oknas are clearly the best window, the improvement over the Vytex windows wasn't ultimately worth the additional $1,300 to me. I probably would have chosen them if the price differential was within $100 a window, but not $162.50.

The vytex window is solid. You did well.

Viola, keep in mind the magnitude and service life associated with this investment. This is a purchase that should last 30+ years, and you have one chance to make the right choice. $1300 over that time period is $43 per year! I think that I could find that in spare change in my couch.
I do understand that $1300 upfront is a harder pill to swallow though.
The Vytex is a fine choice. My main main concern is in your characterization of the dealer. Finding a company with a stellar reputation in this industry can be worth its weight in gold. Installation quality, service after the sale, stability, etc. Not saying that this is the case here, but most review outfits are very "business friendly", so mixed reviews may just show the tip of the iceberg.
If you have not signed off yet you might give that a little more thought. If you have, hopefully everything works out. Perhaps you are making the right choice, just take your time to be sure.
Please do post back with a followup.