Assuming 'vpcsb' is the host and wvdial has to run as root.
irene vpcsb=(root) /usr/bin/wvdial
Unless you're sharing the sudoers file across multiple host machines, I'd suggest replacing the vpcsb= with 'ALL='. That way you won't get any nasty surprises if you ever change the hostname.
You might also want to consider adding NOPASSWD: in this case. So,
irene ALL=(root) NOPASSWD: /usr/bin/wvdial
Given that you've got multiple users you may even want to consider use of a user_alias:
User_Alias WVDIALERS = dougb, irene
WVDIALERS ALL=(root) NOPASSWD: /usr/bin/wvdial