Ok. They didn't put their hand on the bible and swear they they will attack the tankers..No, the US did not make any such vow. Based on US statements they aren't going to stop the tankers.
But they did say they have Venezuela surrounded:
“We’ve got it [Venezuela] surrounded, it’s surrounded at a level that nobody even knows but they know. We are watching to see what happens,” he warned during a conference call with Hispanic leaders.
“We have surrounded him at a level that no one knows, but they know it.” He added, “Something is going to happen because we won’t be able to bear it,” without specifying what “something” he had in mind.
And they did say they wont allow us to cooperate with Venezuela
“The president has made clear the United States will not tolerate continued meddling by supporters of an illegitimate regime,” the official said. The administration considers Maduro a usurper, saying he stole the presidential election in 2018; the Justice Department indicted him in March on narcoterrorism charges. The United States is one of nearly 60 nations that recognize Guaidó, head of Venezuela’s National Assembly, as the country’s lawful leader.
And they have already put sanctions on our oil export, so per their view, we are breaking 2 laws.
1. Cooperating with an illegal regime (Venezuela) Which they have said they wont tolerate.
2. Exporting oil which they have stated several times, they wont allow us to do.
And they have said thousands of times that they will do whatever it takes to stop our oil exports and break our economy.
Now there are five tankers full of oil, and important equipment, heading towards their own backyard, to deliver it to a country they are trying to break.
They said they will do "whatever it takes" Now it takes military action. So they did vow they will use military force on the tankers.