Learn Farsi: اینجا تاکستان نیست، بلکه علفزار است. - This is not a vineyard, but rather a grassland.

In Farsi, when you are identifying or describing a physical location or area, it is much more natural to use اینجا (meaning "this place" or "here") as the subject rather than the generic pronoun این (this).

How exactly does بلکه work here, and could I just use ولی (but)?

You cannot use ولی or اما here. بلکه specifically means "but rather" or "on the contrary." It is used when the first clause makes a negative statement (like using نیست) and the second clause provides the correct positive alternative to replace the rejected idea.

The words for vineyard (تاکستان) and grassland (علفزار) both have suffixes showing a location. Is there a difference between -ستان and -زار?

Yes, both suffixes mean "a place abundant in something," but they have slightly different flavors. -ستان (-estan) is often used for cultivated areas, institutions, or regions. -زار (-zar) is usually attached to natural elements or plants to describe landscapes and fields, such as grass (علف).

In the second clause, does علفزار است need the verb است, or could I drop it because it is implied?

In formal or written Farsi, you should keep the verb است to properly complete the second clause. In everyday spoken Farsi, however, people would either contract it to the "e" sound () or sometimes drop it entirely because the contrast makes the verb obvious.