Turn it into a plus if you can.
The previous owner really needs to give you a list of outstanding vouchers, saying if they are for treatments or for a specific value. Find out if the vouchers typically have a validity ( 3month, 6months, 1year ).
If there aren't that many, and you have contact details for the buyers, you could honour the voucher.. that way they will experience the new salon and could become regulars.
You could offer then a complementary treatment to replace the voucher, ( maybe at a lover value) if there are quite a few unredeemed vouchers out there.
I'm sure it makes a difference if the salon has the same name... or has changed names. If you just took over running the business... then maybe presold vouchers should have been disclosed before the sale!!