Vista's growth is a wonderful thing, but our wish is conflict, isnt it? leave Caster for someone who would be ok with the less savory practices.
also, why you say Mo-tan would not listen to her Master? they can get along just fine, just you see.
also, lets not make OP servant-master pairs. all powerful servants should be hindered by compatibility, so as to not repeat Fuyuki GW in FGO timeline (where Solomon steamrolled everyone).
Yeah, we're (You and the rest are) the Grail, and we have two goals, one to make a Holy Grail War where everything is 'balanced' and able to generate conflict, and to fulfill our purpose, to fulfill a wish of some kind. Also, neither Caster included is with the less savory pratices (maybe Zhuge Liang is, but... He's rational to the point of being a computer.), one's an Author, not a magus, the other's a strategist/magi combo... with the latter part being a Teacher.
The first thing in response to the second, Vista's... 13 or something? and looks a bit younger I think, so right off the bat, Mordred's not going to take her seriously (like everyone else in Vista's coworkers), which will immediately have the two at worse compability than the first estimation. Next, Mordred has issues, and is likely to not heed any of Vista's commands/orders if she cares enough or gets worked up enough as well. Lastly, there's the chance that if Vista doesn't go along with Mordred on several occasions that matter, the latter is
very likely to just kill Vista right then and there, and go look for another Master. That's just from a compability stand-point.
As for OP pairs, the only one that remotely stands out is Waver/Zhuge Liang, Hans takes a
long time to ramp up in comparison to other Servants, Chiron has low attack power compared to other Archers, and his Noble Phantasm has to be used at the best moment possible since it's once per night only, and deals low damage.