A bill applying Israeli law to the West Bank, a move tantamount to annexation of the territory Israel captured in the 1967 Middle East war, won preliminary approval from Israeli MPs on Wednesday.
Asked by reporters about the move, Vance said: "If it was a political stunt, it is a very stupid one, and I personally take some insult to it."
Vance spoke after US Secretary of State Marco Rubio warned that moves toward annexing the territory that Palestinians seek for part of an independent state could endanger Trump's plan to end the Gaza Strip war, which has yielded a shaky ceasefire so far.
"The West Bank is not going to be annexed by Israel. The policy of President Trump is that the West Bank will not be annexed. This will always be our policy," Vance said during a trip to Israel to help shore up the truce.
The opposition initiative for the annexation of parts of the West Bank surprisingly received a majority in a preliminary vote in the Israeli parliament on Wednesday but final approval is considered unlikely.
Ultranationalist members of Prime Minister Benjamin Netanyahu's government have long been advocating for the annexation of the West Bank.
The Palestinians claim the area as part of a future independent state.
Israeli Foreign Minister Gideon Saar said after Vance's remarks that the government had not decided to bring the vote on annexation forward at this stage in order to ensure the success of Trump's multi-stage Gaza Strip plan.
The US has long been Israel's most powerful and staunch major power ally and Trump's administration is particularly close to Israel with considerable sway over its leadership.
Senior US officials including Rubio have been visiting Israel seeking to keep alive the brittle ceasefire between Israel and Palestinian militant group Hamas after two years of devastating war that has upended the Middle East.
Vance told reporters in Tel Aviv that he "feels pretty good" about the Gaza Strip ceasefire after having talks with Netanyahu.
Repeated bouts of gunfire and explosions have shaken the deal and the two sides have traded blame for violations of its first phase, which featured the return of Israeli hostages in exchange for Palestinian prisoners, a handover of bodies of some deceased hostages and a partial pullout of Israeli troops.
Both sides have reiterated their commitment to the US-mediated ceasefire after two years of war triggered by the October 7, 2023 cross-border Hamas assault on Israel that killed 1200 people, according to Israeli tallies.
Israel's air and ground war in response has killed more than 68,000 people and reduced much of the tiny, heavily urbanised Gaza Strip to rubble, Gazan health authorities say.
The US State Department said Rubio was visiting Israel to support the implementation of Trump's 20-point plan and pave the way towards reconstruction, stable governance and possible steps towards Palestinian statehood.
He was preceded by Vance, who was also due to meet Israeli Defence Minister Israel Katz and Strategic Affairs Minister Ron Dermer on Thursday before departing.
Separately on Thursday, Israel's Supreme Court held a hearing into whether to open the Gaza Strip to the international media and gave the state 30 days to present a new position in light of the new situation under the ceasefire.
Israel has blocked reporters from entering the enclave since the war erupted with the Hamas-led attack on Israel on October 7, 2023.
The Foreign Press Association, which represents dozens of international news organisations, had asked the court to order the government to open the border.
with AP and DPA
