To put it simply, building Veigar only to take out the squishies gimps him greatly. Rather than focusing on canceling out their magic resist, I focused on what would be more effective against the tanks. Any good tank will stack 200+ magic resist if there are 2 or more AP bursts on a team so % equipment fares far better than flat pen or resistance reduction.

The only item I would consider possibly leaving till after Rabadon's Deathcap would be the void staff. Completing Archangel's staff gives as much AP as Rabadon's Deathcap would (including the 30%) while being cheaper to finish. Void staff is more for the sake of any toughies/tanks and increases Veig's outgoing damage to any toughies/tanks far more than the RD.

Agreed, I'd take lich bane over death fire grasp almost any game since it allows you to spam it every time you burst someone, even if your ult is not up. Plus it give more AP. about the defensive items, I think any decent veigar with a supporting team can keep himself safe by stunning, bursting and getting the hell out.

The main problem with the build is magic penetration: Sorcerer boots are a lot better against other squishy champions (veigar's specialty) since they give flat stats. If you want CD reduction you should get CDr runes or other items.
http://www.mobafire.com/league-of-legends/build/words-on-magpen-mr-ap-graphs-incl-why-to-play-hybrid-9520
Read link above to get a better idea on how MR and magic pen works.
Also you delay the deathcap too much, remember that veigar's abilities have great scaling and deathcap also buffs all your ap gained by archangel, medjais and your last hitting with baleful, so getting it after the void staff ( which is pretty useless against squishy champs) will really slow down your burst mid game.

Deathfire's Grasp is worthless on a Veigar unless you plan on getting into a fight once every minute. Lich's Bane can trigger once every 3 seconds and will deal as much as Deathfire's Grasp and can make your nuke kill a player without needing your ult. Also, Veigar isn't a champion who is played to go head to head with opponents; his event horizion is his ultimate tool in both going on the offensive and saving his hide from a dangerous situation.

Deathfire Grasp isn't as useful as lichbane.

You have 3 Nukes on veigar already, don't need a 4th.

Lichbane allows you to hit hard inbetween spells.

When you build veigar you should just get standard nuke items. He does not need Lich Bane. A Deathfires Grasp is probably the most useful item for him. I also see a lack in survivability, so try and clean that up. Great ideas though