I can only post my experience.

About 14 years ago, I fostered three 3-week-old kittens, one of whom tested positive to FeLV at 7 weeks. Three weeks later, he was re-tested, still positive, his sister then tested positive and their brother was negative. The positive kittens were re-tested a few more times and always positive. The brother (Ted) tested negative all his life in spite of living with his sister. The first kitten to test positive (Roo) died at 10 months. The female (Seshat) lived for 7 years. After Roo died and she lost her companion, the vet told me that as long as my cats had already been vaccinated against FeLV, and their vaccinations stayed current, it was probable that they would not contract the virus since they were all adults (about 4-8 years old) and had been strays. This proved to be perfectly correct. Seshat lived with the other cats, sharing everything, and none of the 8 cats she lived with ever contracted FeLV. They died of renal failure, heart disease, etc. in their late teens, and Ginger is now in her 23rd year. In fact, none received their booster shot after the first 4 years because some became ill afterwards, and there was an injection site sarcoma scare.

Last year, I fostered nine kittens that came from a very poor home and were in pretty bad shape when they came to me. One had tested positive to FeLV, the shelter didn't test the others. After four months, they were all tested and all were negative. However, their bad beginnings did do harm to their immune systems since three of them contracted FIP while at the adoption center.

Isn't it FIV that can show positive when a cat has been vaccinated against it?

I'm with Jenn and others on FIV and sharing. It doesn't even occur to me to be concerned about my big FIV+ boys mixing with my own and foster non-FIV cats. Calling it feline AIDS makes it sound far more scary and serious than it really is, and is a great dis-service to FIV+ cats who are longing for their own homes.