set -m && bash -c "/bin/foo" works but bash -m -c "/bin/foo" doesn't

[Britton Kerin]

The man page says the options available to set are also usable from
the command line.

I wanted to run a backgroup script in a new process group from vim and
print the PGID.
I ended up with the following:

     :echo system('set -m && bash -c '."'".'((/bin/foo &) && eval
"echo -n \$\$")'."'")

I thought it would be nicer to use bash -m but it doesn't work (simple example):

     :echo system('bash -m -c "echo foo"')

says:

     bash: cannot set terminal process group (31921): Inappropriate
ioctl for device
     bash: no job control in this shell

So it seems bash -m and set -m are not equivalent in this context at least.

Is it safe to assume set -m use this way will continue to work?

Is there a less torturous way to create new process groups and get PGID?

Thanks,
Britton