Access provides autonumber: it is called an "autoNumber" field, surprise, surprise. Also unsurprisingly, it is numeric only...
I would keep the Voucher code separate: The Voucher number is the Autonumber field, the Date field is the TodayDate, and build the voucher code from that using string.Format:
string voucherCode = string.Format("JV-{0}-000{1}", date.ToString("yyyy-MM-dd"),voucherNumber);
I would also recommend using a Guid as the PK field if you will need to retrieve the information once you have created it.
I would also recommend making the voucher code a little more complex: If I received a voucher with that format, I might be tempted to try the same date, but one digit less / more if I was a naughty type who wanted to cheat you...