[vile] backup-style in submode definition

[lists-vile]

I have a few settings that are executed when vile is entering the 'mail'
submode.  The following works fine.

~with define-submode mail
    ts=8
    tabinsert 
    wrapwords
~endwith

If the backup-style is present, like so.

~with define-submode mail
    backup-style=off
    ts=8
    tabinsert 
    wrapwords
~endwith

I receive the following error:

[No match for 'backup-style']
[Illegal submode name: backup-style]

How do I set the backup-style based on the submode definition?

Thanks,
Wayne