Those who are used to Western style cocktails and prefer to drink their alcohol in the form of cocktails should tread carefully in Vietnam. A general rule of thumb is that drinks will be worse than you expect them, regardless of the venue. Even in the States, a bar with live music, a dance floor or other entertainment expected to have worse then average drinks. But in Vietnam, even a nice hotel bar with a long cocktail menu will mess up anything more complicated than a rum and coke - and that's if they have rum in the first place, more often than not they'd only be serving whiskey or vodka.
This should be expected - in a country with a GDP per capita of under 2000 US dollars, liquor from the Western world is an expensive luxury. Those in a position to afford the liquor will be more likely to drink it straight, and have an expensive bottle to signal how wealthy they are to their friends. One of the principles involved in finding good food, taken from Tyler Cowen, is that the quality food rises or falls to meet the quality demanded by the consumers. Expecting to find good cocktails in a place where no one drinks them is akin to hoping to find good Chinese food in a community with no Chinese residents.
And even in the United States, appreciation for well made cocktails only really heated up in the past decade or two
. Vietnam's cocktails might most closely resemble those made in the US in the 80's, "...when artificial flavoring and sweeteners were introduced, and fresh squeezed juices and class liquors deemed "our Grandfather's booze" were pushed to the side."
Still, there are some places that put effort into making cocktails according to modern Western sensibilities. Hiring a bartender who actually knows what they are doing and providing them with fresh ingredients is relatively expensive compared a getting bartender whose comparative advantage is just their foreign language skills and giving them off the shelf mixers, so the bar wouldn't just lay out another cocktail menu and expect people to know that their cocktails are going to be good. They show that they have some really interesting stuff going on.
The above bar, Angelina, is attached to one of the most expensive hotels in Hanoi. They have a few very involved cocktails on the front of their menu that cost between 150% to 200% the price of the rest of their drinks. When ordered by someone not sitting at the bar, the waiter will invite them to the bar to look at how these cocktails are made (it's a drink and a show). These drinks are relatively labor intensive and the process is relatively complicated - and sometimes dry ice is added around the drink for no reason at all. The final result is a drink that both costs and tastes like it was made in NYC. The important thing is that the other cocktails at this bar are also made very well. By showing that they are serious about cocktails the customers can order the cheaper classic drinks off of the menu without worrying that about being stuck with a random green sickly sweet concoction.