What's your opinion on this?
I see foreigners do it all the time even when the prices are already low. For example, an old lady who wants to sell sugar cane drinks for 15k dong each which is only 63 cents, not even a dollar. But the foreigner wants to bargain and bring it down to 50 cents. I find that very disrespectful.
I can understand if the vendor is someone who wants to rip the tourists off by overcharging them, but if you travel to a country, you should learn about the currency first and know what is overpriced or a good deal before bargaining. Also tip the old ladies if the price is too good. It will go a long way for them.