how do i find the right liege to swear loyalty to? what are the best means by which i might, er... serve mi(hypothetical)lord faithfully? what am i to do in the case their highness meets an... untimely end? please send all advice you have my way, anything helps
The easiest way to become Grand Vizier is to gain access to the court through a more humble position, like a normal Vizier or a mere servant. From there you can ingratiate yourself to your lord while supplanting the current Vizier. This is by no means easy but if you're seeking this position you're almost certainly proficient enough in plotting, secret keeping, and subterfuge to pull it off.
In the event your lord meets a tragic and unfortunate end, worry not! The heir is almost always more impressionable than his father and may be more amenable to your suggestions and ideas. If there is no heir then congratulations! You now get to choose the claimant to the Throne you most prefer and leverage power to install him. Ideally this won't become a whole succession war but if it does that's just part of the business.