Originally Posted by biomorph
W kills your mana and the way I see it - in early game - one mana not used to last hit with Q is wasted mana.. The Q first approach is primarily to lower the CDR so you can last hit better. And if you need W to get a good enough CS you should practice last hitting better (with auto attack) and you should not need W to farm. The advantage gained from a fast DFG + Sorc boots and running with 100AP early in the game is that you will jsut run and snipe selected targets at will and get farmed (which can also be done without W) and now you farm players rather than creeps. When your enemies become beefier then your W is starting to do the job of finishing them off.
My main point is that this guide is not making the most of Veigar. If we allow that there is some merit in taking W over Q you should always, always, always take DFG. What the OP suggests is almost equivalent to buying a car without an engine ...
Let's say it's level 10(5-3-1-1) when the Vei A, typical DFG rushing build, get his DFG and Sorc Shoe after Doran's Ring. And he got a Blasting Wand for Deathcap in the future as well.
Let's say he's been left alone and has been farming nicely, having +100 AP from Q.
Total AP: 100 + 60 + 15 + 40 = 215 AP.
Vei A's burst will do:
260 + 220 + 250 = 730 + 559(2.6 AP) = 1289 without DFG.
Now Vei B(3-5-1-1), since he rush for Deathcap, say he's allowed to free farm as well, and since his Q doesn't have as low of a cooldown, say he got only 60 AP from Q.
So he'd have Ring, Deathcap, and Sorc Shoe.
Vei B's total AP: (155 + 60 + 15) * 1.3 = 299 AP
Vei B's burst will do:
170 + 320 + 250 = 740 + 777.4 = 1517.4 damage
Damage differences: 228 damage.
With DFG's active added in, Vei A would 'win' in the burst power, though this is in the best case scenario. In actual game, you most likely won't be using Q on cooldown all the time, so result may varies.
But basically, 2-3 in Q should gives you enough damage/decently low enough cooldown to last hit in lane without heavily pushing it. And with W's extra damage, you will have more presence early game (Q-first build lose out on AP each time you have to poke someone, 3Q-W build only Q people for finishing them off).
TL;DR Deathcap first lose out in full combo-burst, but have better AP scaling and more damage in partial combo burst.