The person issued the voucher needs to buy the ticket but he can buy the ticket for anyone he wants. I once had 3200 in vouchers after my family of 4 was bumped from 2 flights during February break and I bought tickets for several other people without a problem.

The rules regarding these vouchers are not well known by the ticket agents and seemed to change everytime I bought a ticket. If you have any issues just keep trying with new agents and you will eventually get what you want.