Seems reasonable enough to me. I have not been to HCMC but have visited the other places. I did Halong Bay as a day trip (admittedly a long day) from Hanoi and thought that was not unreasonable. I think there's a new highway since then that cuts the travel time.

Four days is probably a bit long to most people for LP. I might swap one of those days for an extra day in SR. But I suppose it might depend somewhat on flight schedules.