Not as … as: không … bằng / như

You know bằng for "as … as": cao bằng, as tall as. Put không before the adjective and it says "not as … as": Ở đây không mắc bằng ở quận 1. It's not as expensive here as in District 1 (in Hà Nội, không đắt bằng).

This is how Vietnamese usually says "less … than". Phrases like "less expensive" have no everyday match: you say "not as expensive as", or use the opposite word with hơn: rẻ hơn, cheaper.

Như works in the same place and sounds a little softer, more like describing than measuring: Sài Gòn không lạnh như Hà Nội. After a verb, the whole thing comes after the verb and its object: Em nói tiếng Anh không giỏi bằng chị. I don't speak English as well as you.

Không bằng on its own means "not as good as": Phở ở đây không bằng phở mẹ nấu. The phở here isn't as good as Mum's.

Put không ai, không gì or không đâu first, and it says nobody, nothing or nowhere comes close: Trong lớp, không ai giỏi bằng Lan. Nobody in the class is as good as Lan.

In writing you'll also meet kém … hơn for "less … than", a later pattern.