The Americans are seeking an unprecedented third consecutive title in Australia and New Zealand and, on paper, are in a totally different league to tournament debutants Vietnam, who are ranked 32nd in the world.
While a tougher match against 2019 finalists the Netherlands awaits in their second Group E game, defender Crystal Dunn (31) said they would not take their eye off the ball.
"We've got to get through this game before we even consider talking about the second game," said Dunn.
"We haven't played against Vietnam, this is a team that we have some footage on but we've obviously never played against them but I think they're going to bring their best game and we know that going in."
US forward Lynn Williams (30) said they would need to be prepared for "a different style of play".
"Vietnam is a little bit of a wildcard," she told reporters.
"It's just (up to) our ability to adapt. Obviously every single opponent that we play, we scout them and then half the time it's not what we scouted so just trust in each other."
