Thanks for the comments everyone. Having found agents at AA, UA and US often mistaken on things or unwilling to take much time to find the right answer, and knowing cs at Priceline would be less helpful, I was looking for some thoughts on the best way to proceed so I could lead the conversation where I wanted it to go. Everyone here was helpful with that. Here is my result from midday yesterday, before the last few comments here were submitted by all of you:

I first called NH. I knew they likely couldn't do much, but was hoping to find out their position at the least and at best maybe get an agent who would do things for me. The former was the case. The Los Angeles phone number answered promptly, and within a minute or two was able to give me the cancellation policy (only taxes refunded) and the change fee (120). He was unable to cancel or make changes, allegedly, and unable to give me an exact amount on taxes. About a 7 minute call.

Following that, I called priceline. I did not plan/look for other flight options, as SGN is my only desired destination this year and I do not have the visa issued resolved yet. I also knew with priceline they couldn't reserve the value in a change, i.e. issue a voucher. So I would have to choose new flights while on the phone. Following a 50 minute initial hold, they took an additional 30 minutes to "open the fare rules" and tell me NH's policy, which I already knew. I did have them price out a change to HKG, for comparison, in the fall. In addition to the change fee for NH of $120, priceline adds their own $30 dollar change fee, which I was aware of. The flights I priced out were an additional $600 for September travel, including the $150 in fees. I decided to cancel, which I had been leaning towards anyway. This took an additional 15 minutes on hold while priceline called NH. In the end, priceline was also unwilling to tell me how much I would be receiving in a tax refund, claiming they didn't want "legal issues with complaints". Call was about 1:45 in length, almost all of it on hold.

Overall priceline agent was kind, but very slow. Results were as expected. NH was prompt, knowledgeable and helpful, also as I expected. The priceline receipt listed my taxes as about 30% of my total cost before I cancelled, so I am hoping to receive approximately that much in my refund, although I know it might differ.

I appreciate the comments, and I understand general suggestions are all that I can usually get on FT. But just for clarity, I don't really know what specifics you mean. The entire set of flight rules? I've given route, date, airlines, fare bucket (I think), and fare basis. I really don't know how to read some of the sections of fare rules, so maybe I need to find some websites that walk me through that.

Also MSP, I did look through the OLTA forum, and found most comments were complaints about the service. I did not do a thorough search, as I suspected NH's policy was guiding, and priceline just adds their fees on top without many additional rules. That might be wrong in general, but it did seem to be the case for my situation.