I don't really see that it needs a special interaction, but I do wish it was still called Frozen Fist just because Vi.
Having her simply play her selection audio "Punch first. Ask questions while punching." when you build Frozen Fist would be the most I'd want to see as a special interaction
Now, trinity force is a huge cost that provides an unreliable slow and ms boost; this will not match the stickiness of Frozen Mallet even with the speed buff. The only place where you should compare Frozen Fist is with regard to your tanky item.
Frozen Fist will be demonstrably better than Sunfire Cape in so far as it provides more sticking, and more upfront damage than the sunfire will match; Every point of armor requires a unit to take 1% more of its maximum health in physical damage to be killed. This is called effective health. A unit with 60 armor has 60% more of its maximum health in effective health, so if the unit has 1000 maximum health, it will take 1600 physical damage to kill it. What this means: by definition, armor does not have diminishing returns, because each point increases the unit's effective health against physical damage by 1% of its current actual health whether the unit has 10 armor or 1000 armor. As you move over 2000 hp, you will get more from Frozen Fist than you would Sunfire, with more overt damage in your burst than sunfire can match.
Randuin's, however, is far better a direct tanky item; it reduces -attack speed- as well as move speed, it is activateable when needed, and means that those trying to kite you will automatically be slowed from striking you.
Frozen Mallet, Randuin's and Frozen Fist will provide you a huge level of tankiness and ability to stick to anything with team utility, but would only afford you, say, Black Cleaver and Blood Thirster (or Blade of the Ruined King if you wanted the active for more stick/stacking with your W). This would leave you with no MR, no GA, and only viable if you were vs an AD heavy team.
But with regard to trinity force, you will get more from a Statikk Shiv for less, especially as you could sit on the zeal + averice blade to stack in gold. Zephr brings you better speed and Tenacity. Ghost Blade has far more impact than trinity by virtue of armor pen, activating straight after an engage or being used for the burst chase, and synergising with black cleaver.
Remember, Trinity Force got nerfed, and does NOT provide more slow than Phage does, nor a higher chance. You are actually better off just sitting on your Phage and building a specialist item for speed than upgrading to trinity force.