MP3 players do not need formatting by Vista for storing music. What may be happening is that half of the player is sectioned off as a USB flash drive and the other half is being used for music files. Check the manual for ways to reduce or remove the flash drive partition. It may also help to use the software from the company that makes the player. That software might have a tool to let you choose the size of music and data partitions.

If you need more help, please include the make and model of your player.

@ScottW Its an unbranded chinese one and doesnt have a meaningful manual.(you get a lot of such stuff in India)

If you look at it in Device Manager (Disk Drives). What name does it say?

Its just calles Generic USB hub