Is it appropriate to haggle with street vendors in Vietnam, or is it disrespectful?  

I think being a part of the culture when it comes to haggling depends on what you are buying. If it is a set price at a store or restaurant, you just don't do it. If it is an open market, you can see if they will come down in price but really, I don't do this myself. I would rather spend more just to support the local community. 

Bargaining is the tradition here with street vendors and in most stores except mini- and supermarkets and fancy store. In fact, if you do not bargain, people think you are silly

I have never tried it. I have mixed feelings about doing it. I just don't want to come off as rude or ungrateful. I am just not used to doing it where I live.

Bargain with a smile !

Ha, I know what you mean! It can be tempting to try to haggle for a better price, but sometimes you just have to know when to call it. Especially if the price is already super reasonable like, come on, you're really gonna quibble over a few cents?! At the end of the day, it's about showing respect to local vendors and supporting their livelihoods.

I think there is a time and place for it. If prices are set, it is rude. If there are no prices or it is an open market, people enjoy making deals. 

I have not participated in it myself but this is mainly down to not wanting to look cheap or ungrateful. I know it is a part of the culture but I would rather pay more to support the locals than try to save money when I am already getting an amazing trip for so little.