Meaning and usage of "It's in that vein"

The phrase, "in the same vein," means "in the same general category or style".

The waitress in the Seinfeld scene is explaining that, "Well, what's it going to be?"; "What'll you have?"; and "Are you eating?" are all phrases that mean pretty much the same thing.

According to the following reference, "in that vein" means "similar, on the same topic, along those lines". http://www.english-idioms.net/cgi-bin/idiom.cgi?idiom=in%20that%20vein

It's not wood...that's grain.

The original phrase is It's in a similar vein, which actually started in mines since ores naturally formed long streaming deposits called veins. Miners would use the phrase to effectively communicate the locations of each separate vein. So if a miner uncovered what looked like two different deposits, a senior miner may come down and tell him that one deposit is "in a similar vein" to the other.

Incidentally, Barrie England's explanation and my own are in a similar vein; The deposits were so named for their striking resemblance to blood vessels.

However, the origin is just fun trivia in case you wanted to explain it to someone else, as it's not a phrase I hear often anymore. At least you now understand that the phrase denotes a relationship or connection between things. In Kelly's case, her waitress vernacular for May I take your order

It means ‘it’s on those lines’. It’s the same vein that describes the blood vessels that return blood to the heart, but the word has accrued many figurative uses.

She's trying to explain that, when she says "What's it going to be?," she means it as a synonym for "What will you have?" or "Are you eating?" When two concepts are closely related, they are in the same vein.