It's been a couple of days and I've spending the time trying out Vi when I've gotten the chance. I'm not quite sure how I feel about Riot's recent tendencies toward weaving new champion lore in with the old (Diana/Leona and Zed/Shen come to mind). Vi is touted to be Caitlyn's partner-in-crime, or partner-in-law as it were. Brash and straightforward, Vi is apparently the "cowboy cop" to Caitlyn's by-the-book policework.
Her kit is fairly reminiscent of Riven, both of them being high mobility bruisers. Vi's strengths lie in her ability to assassinate the carry. Well, not so much "assassinate" as "bulldoze". Her abilities fit her rash personality, centering around moving her about the battlefield with little regard to what stands in her way.
Blast Shield - Vi's innate gives her a shield upon use of her abilities. This shield scales to her health. Honestly, this innate works like a much weaker version of Malphite's innate, Granite Shield, albeit far weaker. While they both shield the same amount (10% of max health), Vi's shield only comes up upon ability use and has a fairly long cooldown. This means that Vi is weak to harass (what top lane bruiser isn't?) and can only trade every now and then.
Vault Breaker - This ability is Vi's bread. This allows Vi to initiate through minions and start duels with her opponent. Considering her innate, this usually means that Vi will come out ahead. Considering its high cooldown, even at max rank, Vi will really only be using this ability to initiate. If it weren't for what was coming next, Vault Breaker would really be considered sub-par.
Denting Blows - The butter to Vault Breaker's bread, this ability is what makes Vault Breaker amazing. Denting Blows deals damage that scales to the target's max health, reducing the target's armor, as well as increasing Vi's attack speed. Outside of being applied by Vault Breaker, Denting Blows also procs upon the third hit on the same target in a manner similar to Vayne's Silver Bolts .
Excessive Force - While Vi's Vault Breaker and innate serve her well in duels, Excessive Force works best as a minion-farming and harass move. While middling at rank 1, you will be clearing entire minion waves at max rank. Be sure to place the enemy champion within the line of fire for Excessive Force whenever you use it and you'll be on your way to winning your lane.
Assault and Battery - Used well, Vi's ultimate can be very frustrating for the enemy team. Used badly, Vi will become fish food for the enemy team to feast on and your team will be 4v5 for the next teamfight. Its key point is the immunity to any incoming crowd control as you speed toward your target. While this means you can easily get to the enemy carry, you will be easily focused down without support from your team.
Vi plays very similarly to another top lane bruiser that we all know and love: Riven. Like Riven, Vi will be focusing on trades and keeping her lane opponent from farming. Like Riven, Vi relies on high mobility and her dueling capability to win her lane. I'll leave off on discussing item builds, ability progression, masteries, and rune set-ups as that is hardly my area of expertise. Just know that you'll be focusing on dueling and trading with your lane opponent. Get a lane advantage and the enemy team will be hard-pressed to stop you.
When you're not trading with your opponent, keep up farm with Excessive Force and make sure to use it to harass your lane opponent as the same. Without significant sustain, your enemy will become increasingly easier to kill in the next duel or gank. Remember to keep your duels to short, quick bouts. Vi's shield doesn't last for long and it doesn't shield for much, so over-long duels will not end in your favor.
Vi, like many melee champions, is prone to being kited. Her crowd control is all instant duration and will not help you in prolonged chases. However, a slow item (Trinity Force , Iceborn Gauntlet, Frozen Mallet , etc.) and judicious use of Vault Breaker will let you catch up to many champions.
Currently, Vi is being compared to Riven in most discussions. General consensus is that Vi comes off lacking, so much so that many believe the upcoming Riven nerf is an attempt to make Vi more appealing in comparison. While I agree that Vi is a far less capable duelist than Riven, it would be remiss of me to say that Vi is patently weaker than Riven.
I agree that some balancing is needed, chiefly Riven's ridiculous health regen (which will be nerfed in the upcoming patch), but here's some food for thought: Riven is a CD-only champion while Vi uses mana. You would assume that Riven would have higher cooldowns to balance her independence from any secondary resource, but you'd be wrong. At rank 1 for each skill, Riven has lower cooldowns. Rather than nerf base ability attributes, perhaps Riot should consider giving CD-only champions higher cooldowns on all their abilities.
Alternately, some buffing could be applied to Vi's innate shield. Its scaling is identical to Malphite's (10% of max health) and yet Vi's shield only lasts 3 seconds and has an internal cooldown. A larger shield (perhaps 15-20% of max health) would give Vi more durability in trades and give her a more equal standing with Riven.
All in all, Vi is a fun champion. Even with her current inenviable title of "Riven with mana", Vi is a great champion to play if you're the type of player to rush into battle with reckless abandon.