As part of my second trip to Vietnam in early 2017 I would like to spend 2-3 days at a beach resort area. I hear that Nha Trang has become overrun with Russians so I'd like to avoid there if that is the case. I will arrive into HCM and leave from Hanoi, traveling north. I plan to visit in the center, Hoi An and and/or Hue for access to Phong Na Ke Bang and the caves.

What is your opinion of Phu Cuoc? Worth the expense and time to fly there for a beach stop?

Phu Quoc is expected to have the best beach landscape in Vietnam: pure water, original sight-view and decent sandground interact with decent beaches. Had to say intensively worth visiting once

You can try Danang, if you visit Hue, so Danang is a destination you should visit after that. With clean beach and so beautiful, you can enjoy you holiday with full expected.

There are some reasonable beaches just outside of Hoi An. Phu Quoc is a long way to go just for 2-3 days.
Da Nang has China beach with some nice 5* hotels but it is effectively a giant construction site these days with lots of new development going on.

An Bang Village, a few kms from Hoi An, has a nice uncrowded beach. I don't think I'd call it a "resort", but there are places to stay and restaurants, no high-rise buildings. The village is very sweet too. You can walk, cycle, or take a taxi into Hoi An.

I don't necessarily have to stay at a resort. Last time I was in Vietnam in 2004 we stayed in Hoi An in the city and took a taxi out to the beach for the day. it might have been An Bang Village, I don't recall the name of where we were. I believe there was only one hotel built out there at the time.

Is this still a good way to have some beach time in the Hoi An area without actually staying right on the beach?

In 2004 China Beach was just beginning to be developed.

We loved the Hyatt at danang

Oops, that posted twice, and I'm wrong about the most recent restaurant review being mine - there are lots of current reviews.

Besides Nha Trang, Phu Quoc will great choice for you. Phu Quoc is known as long-sandly beaches with blue water and luxury resort with amazing services.

besides Nha Trang, Phuc Quoc or Cu Lao Cham in Hoi An are the choices. All of them are good and very nice.