Jothar wrote: “The teretz must be that vashti had 2 mothers. And in fact we see it says ‘Gam Vashti Hamalka asisa mishtei nashim'”
My brother independently came up with a similar joke several years back. In that girsa, it was a dig at Vashti’s weight.
Of course, the problem with this joke is the dikduk. “As’sa” means “she made” – “ne’es’sa” would be the right word for “she was made.”