Generally speaking, you want to either secure kills/assists early with Dark Seal, or you want to out CS the other midlaner to afford more speed items, making you a good ambush character that can run away from most fights when you over-commit or a team fight goes sour.

In early game, just before you hit level 6, or if you are good enough at CS with Vel, you can combo your W and Q, into an E and a W to hit 2 procs of Organic Disintegration, or a Q W E and R and kill your enemy midlaner for early stacks for Dark Seal. Then, once you have those stacks, you either build into an Archangels Staff/Morellonomicon or go Mejai's and roam if they are doing well.

Mid game, you are generally either going to be really well off with a decent number of assists and a couple of kills, or you are going to be breaking even/negative, in which case you may need to build full penetration or go full AP to catch up with the enemies teams scaling.

End game, you generally fall out of usefulness to the ADC or the jungler for DPS, but your ultimate makes you essential for team fights, and staying alive until the enemy team dives yours, or vice-versa. Picks are the advantage of this type of build, and being able to move quickly in and out of combat makes you extremely good at dealing with stray junglers or tanks, or stealing Barron or Dragon with your R.