Four spirits in the Goetia have no counterpart in the Pseudomonarchia Daemonum, and Vassago is the first of them. The other three are Seere, Dantalion and Andromalius, numbers seventy, seventy-one and seventy-two.
The pattern of the addition is worth noticing. Three of the four sit at the very end of the list, appended after the material inherited from Weyer has run out. Vassago is the exception: he is inserted early, at position three, between Agares and Samigina. Whoever assembled the seventy-two did not simply tack four names onto a sixty-eight-name list and stop.
Where the four came from is not known. They are not in Weyer. They are not in the Latin or English printings that stand behind him. No earlier grimoire has been shown to contain them in this form, and the compiler of the Lemegeton says nothing about his sources for any part of the book. Suggestions have been made, that the names are corruptions of entries already in the list, that they come from a manuscript tradition now lost, that they were invented to make the arithmetic work, and none of them has evidence behind it.
That last possibility is the one that would be most interesting if it could be shown, because the number seventy-two is the whole reason the addition was needed. If four names were manufactured to bring a sixty-eight-name catalogue up to the count of the Shem ha-Mephorash, then a quarter of the last four entries in the most influential demonological list in English is editorial filler. It is a reasonable suspicion. It is not a demonstrated fact, and it should not be reported as one.
What is demonstrable is the negative: these four are late, they are unattested before the Lemegeton, and any account that presents Vassago as an ancient or traditional figure is going beyond the evidence.