Early-Game You usually want to focus on farming, preferably single target camps such as red buff, blue buff and gromp / crab. Unlike many other junglers who tend to be in meta usually, You don't have a very strong lvl 2 gank, so if your opponent jungler does, make sure to inform your team about it. On the other side, you do have a very strong dragon and herald take. You can even rush the dragon at spawn (5min) which is what you usually want to do with Vi if the opponent jungler is not around. Make sure to go for herald as well, especially before 14min, through Herald you can take first tower break or just give a nice bunch of gold to your laner very easily.
If you are maxing Q which is the path you wanna go for most of the times, you have to keep in mind that enemy can dodge it and you will lose most of the damage and the very important CC/aftershock proc as well. Therefore, ganking from flank or bush, etc. positions from which enemy does not have as much time to react to you as if you just walked up to him, is rather the prefered way.
You can also do a Q flash, try it out in practice tool, speaking from experience, it's very useful at obvious ganks when enemy is able to react with their flash to you and also if you miss your Q in a situation where you should not be missing it, you can make up for it with the flash trick.
Aftershock keystone Aftershock is ideal keystone for Vi simply because of her R being reliable way to use it, howerver, you prefer to get Aftershock proc with your Q to proc it immediately as you arrive to the fight. It is a weaker in terms of damage dealing than other keystones, but gives you a massive ammount of tankiness. Be careful before lvl 6, if you miss your Q, you are basically fighting without a keystone.
Electrocute keystone Electrocute is better option for early ganks and 1v1 fights but you are very squishy in fights, so do keep that in mind.
Using ultimate You can use your ultimate as a damage source in combat.
You can use your ultimate to pick a single member in the fight and lock him down, using R on someone then Q, after you land on his head, keeps him at a place for quite a long time, which can be lethal especially against squishy carry champions. Be careful of enemy Morgana / Spellshield champions.
You can use your ultimate to zone enemy from your fed carries and create as much chaos in the fight with sacrificing yourself as possible.