Madagascar’s economy is focused on vanilla, while saudi arabia’s economy is centered around oil. why have these countries chosen to specialize in these particular products?

Are there any options?

i see some aspects in common:
both oil and vanilla are relatively expensive and the countries can be sure to sell them. there isn't much competition (there is quite a lot of competition in oil, but the earth needs a lot of oil) and they can be sure that the prizes won't be suddenly lowered.

also, saudi arabia can barely export anything else, since it's mostly a desert.

No they’ve not done that