It's actually a pyramid scheme, that means you start a business selling a product, and then instead of finding just customers, you find also (or instead) new sellers. They again find news sellers, and so it goes all the way down to the ones at the bottom, who are not able to find sellers anymore. But as long as there are people joining the program, it's actually legal. So Herbalife is a legal business (although I am not sure if you may need a business license and work permit in Vietnam as a seller), I would call it unethical.
Usually MLM means people try to sell to their friends and family and their friends and family.
The basic system is not bad, Tupperware is doing a great job for example, but they take care of both, the products and their sellers. Other, like Herbalife, put a lot of pressure on sellers, usually more psychological. You will find yourself easily in a trap once you joined this world.
They may promise you a lot of money (and easy to make, but it's not easy, believe me. So don't quit your job, eat fruits and all the other healthy vietnamese food and you don't need that Herbalife powder.)
That's what other says about Herbalife and MLM:
Scheme Alert:Herbalife Warning Signs of Financial Harm and Deception Abound at Herbalife
Herbalife, one of the oldest and largest America-based multi-level marketing schemes exemplifies most of the classic warnings signs of a financial trap, a pyramid scheme and marketing program that is steeped in deception.
... 2004 settlement resolved a class action suit on behalf of 8700 former and current distributors that accused the company and distributors of "essentially running a pyramid scheme." A total of $6 million was to be paid out, with defendants not admitting guilt.
In a California class action suit, Minton v. Herbalife International, et al., the plaintiff is "challenging the marketing practices of certain Herbalife International independent distributors and Herbalife International under various state laws prohibiting "endless chain schemes", insufficient disclosure in assisted marketing plans, unfair and deceptive business practices, and fraud and deceit". 
Consumerfraudreporting.com:In a legitimate MLM company, commissions are earned only on sales of the company's products or services. No money may be earned from recruiting alone ("sign-up fees"). One must analyze the compensation plan to determine whether participants are paid from actual sales to customers and not from money received from new recruits. If participants are paid primarily from money received from new recruits, then the company is an illegal pyramid or Ponzi scheme.
The FTC about MLM
Hubpages, a personal view:
HerbaLife makes the big bucks not by selling nutritional supplements, but by recruiting folks to become distributors.These distributors buy up a certain lot amount of the company's products, sample them, and try to sell them. More importantly, though, the newly-minted distributors are "encouraged" to recruit their friends and family (and anyone else they know) to become distributors too. And when I say "encouraged", I mean that the company will pretty much twist your arm in order to have you find new recruits. Weekly meetings, HerbaLife newsletters, company-wide e-mails, etc., will all repeat the company core motto until it comes out of your ears: recruit, recruit, recruit!.
Of course, there is an incentive to finding new distributors: a portion of their earnings on products gets kicked up to you. And likewise, part of your earnings get kicked up to whoever recruited you. And so it goes, until the very few people at the top of this pyramid scheme rake in the big bucks while you drive around in your car with big boxes of muscle-building protein powder. It's multi-level marketing (MLM), plain and simple.
So better apply for a regular job. Or even go to a casino (where at least you make yourself unhappy, but not others)