Vanguard Mystic Patch 10.16

Let's be honest: you can 3-star Jayce, Mordekaiser/Nautilus, Cassiopeia, build the best items possible and still lose to SyndraSorcerers/ViktorMech/AstroSnipers/even Blademasters (Xayah + LW or GRB+RFC Master Yi does the trick). And you have, well, nothing to deal with it. Current meta is swarmed with comps with a loads of True Damage, Armor Penetration or CC, which makes armor unrelialble and Vanguards quite weak at the lategame (5-6 rounds).
However, you can still play this one. The problem is the only way you get 1st place is if all your opponents are facing a really unlucky day.
Item composition featured above is out of date, TriHrd in the previous comment is right to suggest Jayce to be built as AP carry (Gunblade + Rabadon + Ionic Spark for example) because in other cases you won't have enough burst damage to get to enemy carry characters while they wipe you out of the table. That's also the reason to build Cassiopeia with items which make her ability burn targets more quickly than with RFC: Jeweled Glove or Rabadon (the hat is better but you can't always get that many rods). Morello is still a viable item because of the popularity of the mech setups.
P.S. Definitely not S-tier comp atm

You cannot win off of the RFC Cassiopeia carry alone; Blitzcrank comps will shred you after they cc lock Cassio, and even if you position against them, AD comps with infiltrators may still shred your units. (Think Dark Star Jhin.) Ideally, if you are running this comp the Dragon's Claw should never be built; instead, build Hextech Gunblade on Jayce as that will provide him with the AP he needs to become a functioning member of the team. In fact, any defensive items on Jayce are never ideal, as Jayce gets tanky through the stats provided by the Vanguard and Mystic buffs, thus the only stat you need to build on him is healing (Gunblade.) RFC on Cassiopeia is secondary, since most of the time the Infiltrators will target her anyway, and if she stays too far back the Blitz hooks will hit her instead. A core item of Cassiopeia is also, in fact, Hextech Gunblade, because of the insane amount of health she can heal with that instead, negating the effects of the Infiltrators targeting her. The Morellos should be dropped on Wukong instead as he already knocks up the entire enemy board with his spell, thus achieving the same goal of board-wide anti-heal in a shorter period of time and leaving Cassio's 3rd slot open for more AP. This comp wins by hitting 3-star Jayce, 2-3 star Wukong, 2-3 star Mordekaiser and Nautilus if Wukong is only 2star, and 3star Cassio for ultimate stat-checking ability, which is what Vanguard Mystics excels at.