re: Shopping for engagement rings, educate me

If you are going Vintage then definitely rock some pawn shops and see if you can pick up something stellar.

Spend the money on a quality diamond, bands are changeable. Some people are a bit freaked out but check out diamonds online (JamesAllen lets you look at the individual diamonds themselves). You will save mad cash (as in thousands of dollars for a comparable ring) and can get a much nicer ring for the price if you do it online. You need to know exactly what you want though if you go the online route.

Things to consider: What kind of shape does she want? (Usually its either round or princess)
From there make sure you get an ideal cut diamond (shite will sparkle way more, and girls love that sparkly shite)
After that carrot and colour. Obviously you want a solid carrot diamond ring but don't just get something huge that looks terrible. Colour is a bigger deal if you have other diamonds set around it but anything near colourless will be fine.
If you do it online clarity will not be much of an issue as you can see individual inclusions in each diamond so just pick the one you want.

You have to figure out your own price range and go from there.