That question is hard to answer mostly because very few Vietnamese Americans were living in the US prior to the Vietnam War. It was the war itself and its consequences which drove most Vietnamese immigration--there was an absolute flood of desperate families, many of whom opposed the communist regime, who came to the US after 1975. The Migration Policy Institute has helpfully collated a lot of data here.

That said, there was no outright hostility, at least by the government--nothing like the second world war. The US was propping up the South Vietnamese government and viewed them as allies, and US soldiers fought alongside south Vietnamese forces. In US public relations, the Vietnamese were friends, friends we were aiding in a time of crisis. Look no further than John Wayne's propaganda 1968 film, The Green Berets. Here is a still of Wayne making friends with a Vietnamese kid. And here is a still of him working alongside Star Trek's Lt. Sulu, since hollywood couldn't actually find a Vietnamese person to play the part.

That said, there was of course a certain degree of racism present in US culture. We've all heard the epithets, but a quick google search pulled up an interesting article on Asian American Vietnam vets' experiences.