I just did a quick search about Vassago on the forums and the most I found was:
What can you tell me about Vassago (to which nobody answered)
And My first evocation of King Vassago (to which nobody answered either)
What I find the most problematic is that I definitely won’t summon an entity for protection or patronage without studying them, knowing who they are, what they excel at, what they’d like as an offering, how to address them, how not to piss them off, etc.
I’m really liking what I’m reading about Dantalion so far, but only because more practicioners are sharing more experiences. I’m interested in both Forneus and Vassago now as well, but I can only find too few details so far 