You said your traveling late Jan - early Feb. TET Holiday is running until Jan 30 for many people in Vietnam, so hopefully your traveling to Nha Trang in February. Most transportation sells out during TET many weeks in advance.

The beach in Nha Trang itself won't compare to Phuket, but there are some nice beaches just south of the airport in Cam Ranh Bay (about 30 minutes south of Nha Trang).

Have fun^