The default groups the original user should be member of is like below:
id uid=1000(name) gid=1000(name) groups=1000(name),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),109(lpadmin),110(sambashare)
The only way I can imagine what happened is that you omitted the "-a" option which should have resulted in adding vboxsf to the existing groups. If only "-G" option is used you have made the user member of the vboxsf group while removing all other groups.
Try Randy's suggestion to give you sudo-rights again first.
If that does not work you might need to use chroot to get access to your installed system and change system files using the appropriate commands.
Anyway, once you have access you need to restore all other group memberships too.
Since this is a virtual machine you might consider re-installing it of course. Since you are member of the vboxsf group you must be able to store important work to a shared folder outside the installation.
Edit: Oops, writing this before reading your last post.