I always though '--force' is a flag to say: "I know what I am doing, let me shot myself".
Even manpage says VDO should overwrite existing formatted VDO volume with '--force'.
create Creates a VDO volume and its associated index and makes it available. If --activate=disabled is specified the VDO volume is created but not made available. Will not overwrite an existing file system or formatted VDO volume unless --force is given.

What happens if you create a VDO, remove the disk, remove the VDO with --force, reattach the disk and try to create a new VDO on it? There would still be all VDO data and '--force' will not work with this change, so user will have to use stuff like dd. Is my assumption correct here?

If --force is interpreted as "let me shot[sic] myself" then there's a bug in vdo manager in not removing the existing configuration referencing the device, unless we agree that is acceptable as part of shooting oneself; which I think is incorrect.

As to the question related to removing the disk, etc. that will successfully work with --force.  It's the presence of an existing entry in the config file referencing the device that overrides the effect of --force.  If there is no referencing entry, --force would act to allow the reuse of the device.

P.S. This issue gives more support to getting rid of the config file (or, until such time, making it non-specifiable) to avoid the cross-config file scenario of this issue.

# vdo create --name vdo --device /dev/sdb
Creating VDO vdo
Starting VDO vdo
Starting compression on VDO vdo
VDO instance 0 volume is ready at /dev/mapper/vdo
# vdo create --name vdo --device /dev/sdb --verbose
Creating VDO vdo
vdo: ERROR - VDO volume vdo already exists
# vdo create --name vdo2 --device /dev/sdb --verbose
Creating VDO vdo2
vdo: ERROR - Device /dev/sdb already configured for VDO use
# vdo create --name vdo2 --device /dev/sdb --verbose --force
Creating VDO vdo2
vdo: ERROR - Device /dev/sdb already configured for VDO use

So VDO no longer allows overwriting VDO device with another VDO using --force. But there is still sentence in vdo manpage that suggests it should be able to do it:
'Will not overwrite an existing filesystem or formatted VDO volume unless --force is given.'

Can you please fix it in this BZ or should I create a new one?

I don't see the need for a new BZ; it should have been addressed as part of this one.
Send it back.

Updated man page.

Giving back as per discussion to more clearly state that using --force will not work if VDO is already running on the device (checked in /etc/vdoconf.yml)

Alright...everyone's supposedly happy with the man page as just changed (at least in relation to this question :-)).  It now specifies that for an 'in use' device (config file contains a vdo using the device) that even --force will not make us use it.

man vdo:
"If the specified device is already in use by a VDO volume (as determined from the configuration file) the create will always be rejected, even if --force is specified.  If the device  is  not so in use but is formatted as a VDO volume or contains an existing file system the create will be rejected unless --force is given."

# vdo create --name vdo --device /dev/sda
Creating VDO vdo
Starting VDO vdo
Starting compression on VDO vdo
VDO instance 1 volume is ready at /dev/mapper/vdo

# vdo create --name vdo1 --device /dev/sda                                                                                                                                                   
Creating VDO vdo1
vdo: ERROR - Device /dev/sda already configured for VDO use

# vdo create --name vdo1 --device /dev/sda --force
Creating VDO vdo1
vdo: ERROR - Device /dev/sda already configured for VDO use

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:3094