This has a lot to do with the quality of the wool that is used to make the vintage wool blanket. The higher quality wool will last longer as will the softness.
Many say that leaving your wool out to dry in the shade / slight sunlight can help them remain soft.
You can also use a small amount of fabric softener on your wool blanket but be careful not to use too much because of the properties of wool fabric, it will soak up the fabric softener and can cause future odors or stains if it’s too heavy or too much is used and will be very hard to rinse out completely.
Remember that the better you treat your vintage wool blanket, the longer it will last and the more soft and comfortable it will remain.