Get from Ho Chi Minh city to Da Nang

Not sure where your ad is - Heathrow ? Possibly Heathrow operates with independent baggage handling firms instead or alongside its own airport authority baggage handlers. These handling firms, as in your ad, deal with ramp and baggage procedures and are separate from check-in / boarding. Almost all airport procedures can be contracted out but still remain distinctly different procedures independant of one another. As far as I am aware Virgin uses its own staff for check-in and boarding and Virgin staff won't be seen tampering with your luggage in loading bays trying to lose it !!

Its Heathrow, but its the same for Gatwick and probably all the London airports.

It amounts to the same thing though. Whether the airline pays for the services of the airport authority baggage handlers or the airport authority allows tenders to private firms for baggage handling, all bags checked-in are at the mercy of baggage handlers who are not airline employees.
The only airlines that I can think of that could possibly employ their own baggage handlers as staff, are airlines that actually own an airport, for example Koh Samui which is owned by Bangkok Airways. Otherwise its ludicrous to maintain that an airline has "mishandled" your bag even though they take responsibility for any mishandling.

BTW as a matter of interest @herbie47 have you also had your bags mishandled / lost alot ?

No I have not had many problems. I have had 1 suitcase delayed by about 4 days, that was KLM who handled the situation very badly. Easyjet I had a case damaged in transit, that was also handled very poorly, so much hassle to get compensation, really they are mad wasting so much time over a small amount, never flown with them again and that was about 15 years ago. Its interesting what you say but it seems some airlines are better than others. About half of cases are lost or misdirected because of transfers. My wife had 1 case lost by Alitalia, claimed on insurance. It's not so much that it's lost or delayed its how the airline handles it.

Unfortunately more often than not, punters hunt around for the cheapest tickets and expect the highest level of service. Tickets are cheap for a reason, with the airline providing virtually no customer service whatsoever. Tickets with higher price tags are generally from full service airlines commited to customer service. You pay your money and you make your choice.

Best airlines are generally Middle East and Asia. The only decent European one I have flown with is Virgin. American airlines are probably even worse although I have only used them twice.

It sounds like you've done pretty well if that's the extent of your baggage problems. I've had more mishandled bags than that in just one year. But, as a high mileage flyer, I get a luggage allowance of 3 checked bags at 70 lbs. each and I often use all of it. And, I usually try and book the shortest allowable connection times. So, anytime my inbound flight is late it's not a shock that those heavy bags don't make the connection. Still, it's the AIRLINES fault since they set the schedule and the connection time. I've also had bags left behind because the people at check-in didn't get around to putting them on the belt. The women will often wait for a man to come to lift the heavier bags. The bottom line is that it's almost always the airlines fault and blaming others is just wrong.

You poor thing !! but why do you repeat the same mistakes over and over again ? If you know your bags are heavy why don't you ask for them to be put on the belt ? If you are not making the connections why do you keep booking them ? You seem a bit naive, maybe try a different airline to UA and you never know, maybe your luck will change.

BTW your experience is heavily biased to UA, you might want to think twice before you class ALL airlines in the same boat.

I guess you don't understand how airline loyalty programs work. There are dozens of airlines in the Star Alliance and I've flown most of them not just UAL. You seem to think it's my job to be sure the airline does theirs, I don't think so. If the check-in people can't do their job then they need to be retrained or replaced. As a practical matter, I don't much care if my bags are delayed. They usually get delivered to me the next day which is actually easier than me slepping 200 lbs. of luggage through the airport after a long flight anyway. But the fact is that you can't count on delay free travel and those delays are usually directly or indirectly the airline's fault.

There is an airport in Da Nang city. You can take the Vietnam airline (more expensive) or Vietjet air or Jestar airline (more delay :))) from HCM city to Da Nang. You should depart 2 hours before your departure time, from dist1 to the airport only 40-50 mins
By the way, if you want to use Da nang airport transfer, i think you should see http://vmtravel.com.vn/tours/da-nang-to-hoi-an-tour/ , only 15$ for transport Da Nang to Hoi An.