The Polish Wikipedia about the battle of Kowal (1327) says (translation mine, maybe in some places duke and prince are not used correctly):
In 1327 Władysław I Łokietek [the Polish king] begun the process of subordinating the [Duchy of] Masovia from Poland. The successful process was giving to Łokietek a chance of unifying subsequent Polish lands, increasing access to Teutonic [Order] boundaries and improving relations with allied Lithuania. The divided Masovia was then ruled by: Rawa [Mazowiecka] prince Siemowit II, Czersk prince Trojden I and Płock prince Wacław, who on January 2, 1326 in Brodnica signed a defence treaty with Teutonic knights. The treaty was directed against aggressive and claim-ful politics of the Polish king and was to guarantee by the Order the independence and integrity of Mazovia. The fact of negotiations with the Teutonic Knights was not accepted by Władysław Łokietek. After refusal of homage to the Polish king by the rulers of Mazovia, in July 1327 Polish troops invaded the Płock principality of Wacław, while the Lithuanians of Gediminas - lands of Siemowit II. In accordance to the treaty, the Teutonic [Knights] came with an assistance to the Płock prince.
(The war was eventually won by Poland, but Masovia managed to stay independent).
Of course I understand that such a type of war was common those days, and there could have been many different casi belli, but the question is whether a king (or other ruler) could demand a homage from anybody?
The Wikipedia article about vassals is very brief, so is Britannica article, but also other sources do not say it was obligatory to have a senior (lord) and the system is shown as some sort of a contract.
(Please do not include examples where a ruler became a vassal because of a lost war).