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.
It's important for your career progression to maintain an open mind to assassination and treachery. I know that people look at this with some distaste but quite frankly these are the realities of the grand vizier job market. There's only room for one grand vizier, and why should you let something like your rivals better aptitude for forbidden texts or deeper coffers of gold hold you back?
If that's not your cup of poisoned tea, I would recommend becoming skilled in whispering slanderous rumours to the right courtiers to discredit the others trying to usurp your rightful position. If they would reflect poorly on your liege's reputation, they would surely be dismissed!