I upgraded from ubunutu 16 to Ubuntu 18.04.1 desktop (what i really did was reinstalled ubuntu, because i kinda destroy my system) yesterday, and now i want get vagrant back up and running, the problem is that vagrant doesn't recognize the plugin disksize any more. It is installed as you can see (with vagrant plugin list) but vagrant doesn't see it.
keep on getting
* Unknown configuration section 'disksize'.
I also tried installing a older version of vagrant (1.8.5) but it looks like that doens't work with the new virtualbox (doesn't recognize the provider).
i saw the post Vagrant: * Unknown configuration section 'disksize' but that is the way i installed disksize so that doesn't help.
if more information is required let met know.
Today I updated both virtualbox and vagrant to their latest versions. Had the same problem like you just different plugin.
After downgrading vagrant to 2.10 it works again, no issue with virtualbox (no need to downgrade). Hopefully vagrant will fix this with its new release.