I agree wth Bonk!----Nha Trang is becoming a bit of a jungle but there are things to do. Mui Ne ( personally), I think is average( certainly quiet)---Mui Ne has no airport so you have to catch a train ( recommended) from HCMC to Phan Thiet, 4+ hours , train is new, German built, opened late April, 2006, about $10-5USD / person---you can go by bus or hire a car ----Allow $70-$90USD one way, but train is more comfortable and scenery is nice --get upmarket ticket. Get bus or taxi from station at Phan Thiet to Mui Ne (30klm).
With a week you could consider flying to Nha Trang, staying 3 days and catch a bus to Mui Ne ( OK) stay 3 days, then train/ car/bus to HCMC.
Its all a bit personal--up to you.