Am going to be in Vietnam in couple of weeks and need moderate price (under $100) but well situated hotels for us and my parents. Old Quarter in Hanoi and central location (not sure where) for very short stay in HCMC. Any suggestions? Diamond Elegance that has been mentioned a lot has no availability.

There are several Elgance hotels; have you checked them all?I've used www.Agoda.com with satisfactory results. It's the Asian affiliate of www.booking.com, both controlled by Priceline. Agoda has a large selection and a quick glance suggests that prices are even lower now than a year ago; boutique-style midrange hotels at $50 US.

Thanks. Am looking at the Essence in that chain. Any suggestions in HCMC?

We stayed at the Elegance Diamond in Hanoi, and at the Liberty Central in HCMC. The latter is fine, very comfortable, but lacks the charm and personal touch of the Elegance. However, it's clean and comfortable, modern, and in a fantastic location right down the block from Ben Thanh Market.

Check out Maison d'Hanoi.
Rooms are small but clean.
And the hotel is in a great location.
When I stayed there in 2010, the hotel served an excellent buffet breakfast.

Looked at Liberty Central - a bit too expensive for us. Looking for something under $100 in HCMC - any other suggestions?
PS Booked Hanoi Essence (part of Elegance chain), thanks for the help!

We stayed at the 3 Nagas in LP in 2007. It's a little over your price range, but very well located with a good restaurant and nice accommodations. Checking prices--$109 for twin beds, so not too far out of range. They were in the process of changing owners when we were there and wanted input on what needed attention. We liked it there.
Also, if you're at all into weaving or textiles and natural dyeing, check out OckPopTok in LP. They have half or full-day classes which are awesome!

Blue Diamond in HCM, good location by Cho Benh Thanh, quite central to easily walk everywhere.