Voucher remains. If the refund occurs in the next year it will take you longer to get to your next voucher as the refund gets added on to your required spend for the next voucher in the next year. If the refund occurs in the same year as the voucher then nothing happens. See these threads:
http://www.flyertalk.com/forum/briti...1-voucher.html
http://www.flyertalk.com/forum/briti...n-voucher.html