Before using reward points to stay at a hotel in Vietnam, however, I'd suggest evaluating your options for paying and staying at a local hotel. Lodging in Vietnam is incredibly cheap (you can actually get a room for as little as $3-$4 per night, but I wouldn't recommend it). Air conditioned rooms start at around $10-$12 per night depending upon location (cheapest rates are ususally at the small family-owned hotels) and standard hotels can be had for $20-$30 per night (I'd say these are the equivalent of Holiday Inns / Ramadas / Clarions / Wingate / etc - certainly not Hilton quality, but nice). In Hanoi, we stayed at the Hoa Binh hotel and it was quite nice, but not luxurious. Almost all hotel rates in Vietnam will include a free breakfast (typically buffet with some cook-to-order stations). There are a lot of places on the net to book Vietnam hotels, here's one:
http://www.saigontourist.com/.
It's interesting that you bring up this point b/c I was seriously considering staying at the Win Hotel for around $30. (I've read that it was very popular, and can't figure out how to book in advance since there's no email, but will try the site you suggested. I've seen tons of those booking sites, and don't know which to trust.) I was actually thinking of saving my HH pts for some trip to the US southwest at some point b/c I think they'll 1. go farther and 2. hotels here are more expensive.
That said, it's our very first trip to Asia, and I thought that I would need to stay somewhere somewhat western for the first night or two--although I plan to stay in very local places for the rest of the trip.
Anyway, many thanks for your post and the others.