The character in the evidence provided by Eiso seems to be a variant form of 𠰾.

There is a ⿰口尻 in 四聲篇：


泰和五音新改併類聚四聲篇

In https://kirara0048.blogspot.com/2016/11/5_8.html is guessed that 苦托切 is may be 苦㧌切 or 苦挑切.

It became 𠲙 in (some versions of) 字彙補 and 康熙字典：

字彙補 彙賢齋刊本

續字彙補 和刊本

康熙字典 武英殿刊本

元音統韻本字彙補 is correct:

字彙補 慎思堂刊本《元音統韻》所附

The Vietnamese is clearly not cognate with 𠲙 (U+20C99) and the shapes are not identical. Better to keep separate.