I got a vaginal tear on perineum from rough sex. Now the scab fell off and I still have a tear in the skin. Will it heal on its own or need stitches?

Odds are it. Will heal on it's own as it rather uncommon to suffer a laceration from rough sex enough that it requires suturing. You should try to avoid any intercourse or penetrating vaginal activity. Try to keep the area clean(but with something like gentle soap and water with limited scrubbing). If its starts becoming painful, red or with discharge seek a formal evaluation.
Too late to stitch. If the scab has fallen off from the tear, this means it is a few days old. It is too late for the stitching, and you may not need any stitches. Still it is advisable to consult the GYN and make sure that it is not getting infected.
Depends. Small tears will heal on their own. Larger tears sometimes require stitches. Stitches are best for fresh wounds and used less commonly for wounds that are a few days old. Its hard to give you a perfect answer without an exam. Best wishes!