Ah, it's challenging trying not to repeat words D: I would change the sentence into "I'm contacting you about your job vacancy," or "I'd like to apply for the job you posted about in the newspaper." but I see nothing wrong with using the word announcement here.

Maybe you can use "offer", i've seen a lot of job sites with this term.

"Is this sentence correct?" First of all, the phrase you need is "I am writing in regards to..." You can use "advert" in place of advertisement. Other options are "the position offered", "the position advertised", "your job listing" or even "your job posting". Unfortunately, "announcement" isn't exactly the right word in this context. I checked various phrasings of your sentence, and that word just doesn't come up.

- "I am writing " instead of " I write you" - Job Notice could also be used - the vacant position should in the sentence as the company may have more than one advertisement