Vacuuming alone never gets it all — hair weaves into carpet fibres. Rubber, friction and damp are what release it. The methods worth using, in order.

Vacuuming alone will not remove pet hair, because hair weaves itself into carpet and upholstery fibres rather than resting on top of them. What releases it is friction and static — rubber, damp hands or a pumice stone — after which the vacuum can finally pick it up.
Pet hair has a microscopic scale structure, like tiny barbs, that catches in woven fabric. Add static from a dry cabin and it effectively anchors itself. A vacuum pulls air across the surface; it does not pull hair out of a weave. That is why people vacuum three times and still see hair.
A rubber pet-hair brush, a squeegee, or even a clean rubber glove dragged firmly across the fabric. The friction generates static and the rubber grabs the hair, rolling it into clumps you can lift off by hand. This is the single most effective tool and the cheapest.
Water reduces static and gives the hair something to cling to. Lightly damp, not wet — soaking carpet creates a different problem.
Used gently on carpet and floor mats, a pumice or hair-removal stone abrades hair loose from the weave. Use light pressure and never on leather or soft-touch plastics.
Once the hair has been lifted out of the weave, a vacuum with a brush head will take it. Vacuuming first is largely wasted effort.
Seat rails, between the seat and console, and under seat edges collect hair that nothing reaches. Blowing it out into the open, then vacuuming, is far quicker than trying to extract it in place.
A washable seat cover or a fitted cargo liner is the only genuinely effective prevention — it is far easier to shake out a cover than to de-hair a seat. Beyond that, a rubber brush over the seats once a week takes two minutes and stops hair working deeper into the weave, which is what makes it hard to remove later.
Heavy embedded hair, particularly in older cars with worn fabric, is genuinely slow work — often an hour or more on its own, which is why most detailers price pet hair removal as a separate add-on rather than including it. If the hair has been building for years, or the car is being sold, professional extraction and steam will get results that hand tools will not.
Our full interior detail includes extraction and steam cleaning; heavy pet hair is quoted separately depending on condition. See what's included in a full detail or our pricing.