Jim,
You are correct about the slight but noticeable differences. That's what they are. However the reason I have the Promaster and am wondering if I keep the Viper is that the Promaster offered the same noticeable optical improvement of the much more expensive glass, but at similar cost to the Viper. If the Promaster sold for $1,000+ I probably would not have bought it. I accepted long ago the fact that I am a cheapskate at heart, and place far more credence on value for the money than I do on absolute technical superiority. I got the value for the money, based on my assumption that Promaster will last as long as the Monarch I have. Time tells that tale.
And if eye relief is a consideration, then the Viper does have the Promaster bested. Since I have no need to use glasses with binoculars, I sometimes tend to overlook that point. The locking diopter on the Viper is nice as well. One of the real virtues of the Viper is that it has the same sort of a set-it-and-forget-it utility you often hear assigned to an IF glass. The Viper does tend to stay where you put it and neither the center bridge, focus wheel, or diopter have much tendency to move around, unless the user moves them. A little better, I will allow, than the Promaster, but not much. As I noted above the Viper does seem to have a deeper field of focus. The real optical difference is that an 8x Promaster will show a touch more detail "way-out there" than a 10x Viper. The next difference is the FOV. I think the Viper is fine at 347'@ 8x. The Promaster at 393'@ 8x is a littler nicer.
The Viper would be a good glass to hybridize with the Hawke/Promaster. Either give the Hawke/Promaster the strong points of the Viper for an open bridge design, or the FOV and optics of the H/P to the Viper for a traditional design.
It will be interesting to me to see how (or if) the rest of the market responds to whatever level of challenge the H/P glass provides.
I haven't seen the Hawke, but am relying on Frank D's assessment of the strong similarity between the two. I feel fairly safe here because it seems to me Frank and I tend draw the same/similar conclusions on optical quality. But Jim, you are right, I cannot offer a direct Hawke/Viper comparison.