Client Dinner in Denver
This isn't Asian, but from what I remember when I ate there many years ago, Vesta Dipping Grill was elegant, interesting, and unique. Check it out: http://www.vestagrill.com/ You could get one of their really large round booths that has high sides and is very private, and very conducive to conversation.
I've also heard great reviews about Ming Dynasty (kind of cool divey place), Parallel 17 and Domo (though I have not personally been to any because... I am single and have one other foodie friend who is also an ex and well, that is all). Bara Sushi in the tech center is a gem and something to consider Japon in Wash Park is good. They have a lobster/cheese appetizer that is completely to die for and will put a good million pounds on your frame. Oh my, it is so worth it however. :-)
Though it's been a few years, I'd recommend Ming's Dynasty on Mississippi, just east of Colorado Blvd. It is quiet, mid-scale, and the food is excellent. The theme is Chinese, not Vietnamese, but there is a bit of cross-over on the menu. I know that Bill, the owner, will take reservations, and if you have any particular dishes, will likely be able to accomodate your request, within reason.
That is a tough one. My thought for upscale asian would be Sushi Den. It's elegant, but not particularly quiet.
Parallel 17 could be good (as the previous poster mentioned).
The problem with both of those places is they don't take reservations for groups smaller than 6.
A place called L'Asie Fusion Bistro opened in the old Emma's space on 6th Avenue. I haven't been, so I cannot speak for quality, but it looks serene inside (from what I can tell driving by at 40 mph)
Thats a tough one. Denver could use one of those!
Parallel 17 is upscale vietnamese but not quiet and small.
Maybe Domo- not sure I would describe as upscale but very nice setting and great Japanese food:
There is Kyoto Asian Fusion but its not near downtown. On Santa Fe:
There is PF Changs downtown and a Little Ollies in Cherry Creek North which is nicer then it sounds: http://www.ufeedme.com/littleollies/files/info.htm
Or Imperial Chinese on South Broadway (not to far from downtown) This might be the closest to what your are describing if Chinese is ok:
There are several japanese/sushi places that I think you could probably do on your budget but not sure thats what your looking for.
Maybe others will have some better ideas.