HCMC to Nha Trang in a van with driver and return costs will be very expensive. However if you do want to do this I would start in HCMC travel to Vung Tau and along the coast to La Gi, Ke Ga, Phan Thiet and on to Ham Tien, breaking the journey with overnights along the way. You may not get to Nha Trang but you will have a great VN coastal experience.
I suppose its up to your budget. Accommodation along this route will not be very much, maybe $15 max per room and as it's not a very well travelled route you will be a novelty.