If vampires actually existed, what would they need to do stay under the radar, with no one noticing that they don’t age or die?

In the modern world it would be largely impossible for any significant population of them to;

1. Be able to feed on humans

2. Have none ever be killed, even by accident

3. Live in any semi-developed country

4. Avoid creating any tangible evidence of their presence

…all without ever being discovered.

If there was like a dozen or less of them living in remote reaches… maybe. But them existing secretly in a modern developed society and being able to feed on people would be effectively impossible