I've been trained by earlier versions of Diablo to consider vendor items basically worthless once you start finding your own gear. But since I've noticed that a number of players showing their Inferno gear have blue items, it occurs to me that this might be a mistake. Is it possible for vendors to have items that are a high enough level to generate very high stats (like +160 to primary) that would be worthwhile for a higher level character? Or should I continue to ignore them (except in the rare case where they have a yellow for sale)?
Absolutely look at the AH first, where you can easily sort by buyout and pick item traits you want. Often you'll be able to find something much faster, fits your build exactly, and cheaper than checking out vendors.
Having said that, magic find items are horribly over priced right now so I always check out the vendors (at least on inferno) to try and find upgrades for that, especially MF amulets and rings.
Yes, you absolutely should - not just for yourself on the occasion a weapon with decent DPS might appear, but also for the times when vendors stock magic rings or gloves with high primary stats and boosted Attack Speed. Not only are these great for personal use, but they're also often sellable on the auction house for more than their vendor cost.