Depends, how much do you want to juggle eleventy things that should have been addressed yesterday if you end up getting this job?

It's not necessarily a red flag, but it may be "yellow". I would prepare for the interview by having intentional questions. You don't just want to... ask why the position has been opened for 1 year. You want to know what the priority will be in the first 30-60-90 days. Are there outstanding projects or deliverable that have been on the back burner? Was there someone else on the team managing urgent matters or projects that will transition over to the person that is hired?, etc. more

It could be phishing for someone they like or anything..

It can't be vacant whilst you are there you seek to get pragmatic answers because it sounds as exploitation

Ask why it's been vacant. If it's what you can adjust with

I've been in my job for five years, and when I first got there it had been vacant for a year. It was vacant for so long precisely because it wasn't... urgent. Ask the hiring manager, and if you can ask other employees  more

You can ask why it's been vacant for a year, or something similar to it .

Red Flag. Once you're there, everyting that's been waiting will become urgent.