"Phó tiến sỹ" does not exist in some countries. However, in some country there is a degree called "licentiate" which student are also enrolled in PhD program, finish all courses and their dissertations are as half as doctoral dissertation in term of scientific significance or contribution.
But in some countries, people use "licentiate" to refer to pre-doctoral students who are eligible to be admitted into PhD programs.
Based on my experience, I'm a PhD student and I also talked to many scientist in academia about academic system, I think the best translation for PhD candidate is "nghiên cứu sinh".
And thanks very much for your informative comment :)