When I used vouchers I think a year and a half ago, it wasn't possible to buy something with a voucher, if the money of the voucher wasn't used totally. An example: paying € 395 wasn't possible just with vouchers, I had to pay with three vouchers of € 100,-- and € 95,-- with PayPal. I couldn't pay an amount of € 95 with a voucher of € 100.
I found this lines on the website of VSL : Please note that the voucher needs to be used in full in one transaction, so be sure the basket total is higher than the value of the voucher. Several vouchers may be used in one transaction by entering all voucher codes consecutively into the code field. Vienna Vouchers cannot be replaced or exchanged for cash. See https://vsl.co.at/en/Instruments/Voucher