If you're vs AP or your team is behind while you're ahead, your next item after Ionia's and WotA needs to be abyssal. This pushes your mid-game damage to absurd levels.
If your team is about even with you, or you're behind, Zhonia's. This lets you bait and also persist through a fight while outputting damage, hopefully letting your team load in some damage while you clean up after almost everything is on CD.
Unless you can afford Void or you desperately need it next, grab whichever of zhonia's/abyssal you didn't grab before. In some cases, Rylai's will fit in here first. (Morde, fed Garen, Darius, etc)
Your ult at lvl 2 and before some srs AP/magic pen does pitiful damage. Your unstacked E is almost as powerful.
Its mostly there for the damage amp and to scare people. Most of the time you'd get a kill with it, you'd also get a kill without it. However, it definitely gets people to back off after you cast it. Use it to disuade aggression. Before you have your CDR core, don't spam it needlessly. After that, though, it'll be up every fight.
Vlad is all about attrition. With 30% cdr (runes, ionia, wota) and abyssal, if you can't burst people 30% of their HP with Q, E, grab a rylai's and chase them down. Abyssal, Zhonia's, Ionia, WotA, Void, Deathcap/rylai's is pretty much going to be your build most games. The variation comes in when you build it, and that's what sets experienced Vlads apart.
Always focus on farm. Only harass with Q if you know its 100% safe to do so. You need to hit 7-9 without being set back.
Also, in most cases, TP is trap. Ghost/flash or flash/exhaust is most cases. Otherwise your teamfighting won't be stellar in this age of hyper-mobility. Always Exhaust vs AD or 'all-in' champs.
Here's the kicker, though. Vlad does best mid now, not top. While you still can do well top, you may set your team back if you don't take teleport and the other laner does. If you DO take teleport, you might still not have enough of an impact with it. In addition, taking TP makes your team-fighting less reliable, and doesn't let you be as aggressive or reactive against counters, should they arise. Top you are much more susceptible to being ganked, and even dying during a dive.
Mid gives you supreme safety and a very reliable ability to farm (since abyssal both prevents you from being able to be killed with WotA mid, gives you a large damage spike for teamfights [dragon], and allows you to 1 shot casters with stacked E).