I've only used points.com to exchange "orphan" miles/points when they are so "small" that you can't get any type of award for them. I wouldn't use them for significant amounts since the exchange rates are really bad. The most I've used them for was when you got EBay points for Priceline hotel bids, I swapped out the EBay points to AA miles.

Bonnie - if your goal is to get to Vietnam - I think US miles might actually give you more options to get there than DL - since with US you can redeem on United (probably easiest), Singapore, ANA, Thai, Asiana while DL miles you're limited to DL, NW, Korean, and Singapore.