No.
A vassal can only have one master at a time. You are not allowed to vassalize a nation that is already a vassal because it cannot royal marriage you (or anyone else) while it is still vassal.
And since royal marriage is a requirement for vassalization...
You need to figure out another way to make a vassal (Ragusa) and its master (Hungary) break their friendship. But I have seen that the Hungary-Ragusa friendship is not very solid. Hungary usually loses Ragusa's confidence over time, so this is a good thing for you.
You can always military-annex Ragusa, right? Unless you're kinda afraid of the BB it will bring you, since it's a small innocent Catholic country.
