This is general advice for travellers. Exceptions may apply to business travellers or those with special needs:
• Clothes that need ironing: you will not have time, nor will you find an iron or electricity to polish your outfit. Also, high heels will just take up space with little added benefits. By the way, who wears high heels on a trek?
• Large size shampoos, bathing cream, mouthwash, toothpaste and you-name-it. Unless you are really particular about what to use, you can always buy these when you get to Vietnam
• Sleeping bag: they are bulky and unnecessary for Vietnam’s weather condition. Opt for a sleeping sheet instead, or a beautiful one made of silk that can be bought once you are in the country.
• Jewelry and valuables: scream "Rob me! I'm rich."
• Raincoats: in Vietnam raincoat is often very thin and light, and it is available dirt cheap in every single corner stores. Umbrella is not known to be expensive either.