When I add myself "vboxusers" group I an no longer in the "admins" group

Your second command is right.

By default, usermod -G replaces all supplementary groups the user is in (the primary group is generally named the same as your username, and specified separately; don't mess with that though). By also using -a it appends the groups you give.

So this should work (and does, for me, just tried it):

usermod -aG vboxusers myusername

If running that command takes you out of any other group then you've found a bug. Note you have to relogin to see the effects.

Alternatively:

gpasswd --add myusername vboxusers

According to the usermod man page the correct command to add a user to a group is

sudo usermod -G group -a username

or in your case since you want to add your self to the vboxusers group that will be

sudo usermod -G vboxusers -a $USER