How would you define the state of being a vassal?
While I would not say so, there has been a curious, and legally defined relationship between the United States and Japan from the formal end of the war.
By virtue of the Treaty of Peace with Japan signed in San Francisco on September 8, 1951 (effective April 28, 1952), ending the state of war between Japan and most of the Allied powers except the Soviet Union and the People's Republic of China, and the Mutual Security Assistance Pact between Japan and the United States, signed in San Francisco the same day, Japan essentially became a dependent ally of the United States, which continued to maintain bases and troops on Japanese soil.
After Japan regained sovereignty (1952) it remained completely dependent upon the United States for its defense against military threats, as had been the case since the surrender at the end of the war.
...under Article 9 of the United States–written 1947 constitution, Japan forever renounces war as an instrument for settling international disputes and declares that Japan will never again maintain "land, sea, or air forces or other war potential."
Despite the formal rejection of the very notion of maintaining a military, as time passed the Japanese created a defacto military, while defending the notion with the sophistry that the JSDF (Japanese Self Defense Forces) was, because it was intended only for defense, not a military. This was done with the explicit encouragement of the United States. More recently the Japanese have reorganized its defense forces, from what was a subordinate organization to a cabinet level Ministry of Defense.
Presently the Japanese are re-examining Article 9 of their constitution.
In May 2017, Japanese Prime Minister Abe set a 2020 deadline for revising the Article 9 of the Japanese Constitution, a clause in the national Constitution of Japan outlawing war as a means to settle international disputes involving the state.
All quotes are from the following wikipedia articles:https://en.wikipedia.org/wiki/Foreign_policy_of_Japan#Post-occupation_Japanhttps://en.wikipedia.org/wiki/Japan_Self-Defense_Forces#Early_development