Every Monday we will post an entry that hasn’t yet been published with a view towards harnessing the collective onomastic power of the internet. If you have any thoughts about the name’s origin, other variants it might be related to, other examples of its use, etc., please share them in the comments! If you wish to browse other Mystery Monday names, there is an index.
Today’s name is a mysterious Polish/Ukrainian name, Vyrzbantha.
Our example comes from Latin records in the Ukraine, in the 15th C; but a google search shows another Vyrzbantha who was “castellanus Poznaniens” in 1306, so just over the border in Poland.
The context of both instances makes it pretty clear that it’s a personal name, but it’s not one that we’re familiar with at all; and it may be a byname element rather than a given name. We’re hoping someone who specialises in Slavic languages might recognise this and give us some pointers! Please share in a comment any thoughts you might have.
Dictionary of Medieval Names from European Sources 
I’m not a specialist, but Taszycki’s “Słownik staropolskich nazw osobowych” gives the normalised Polish form as Wirzbięta in his supplementary volume (#7). It seems to be a diminutive of wierzba “willow,” and it looks like it was used as a given name, at least in the past.
SSNO volume 7, p. 289 sn. Wirzbięta has (without his references):
Present(e) … Wyrzbantha de Malossow 1365
Nobilis viri Wyrzbanthe, heredis de Xangnicze … ipse Wyrzbantha 1375
Present(e) domin(o) … Wyrzbantha de Branicz, burgrabio castri Cracouiensis 1402
Stanislaus de Kerz cum Wirzbantha de Chmelyk 1409
Excellent! Thanks.