How can I respond to a prompt in os.system?

I'm trying to do something like this:

from subprocess import Popen
p = Popen(["vagrant", "ssh", "vmname", "-c", '"pvcreate -ff /dev/sdb"'])

But it requires user input. Also, that didn't work anyway. They give the error: bash: pvcreate -ff /dev/sdb: command not found, because it's looking for a program pvcreate -ff /dev/sdb, instead of pvcreate with arguments. I also tried this first:

p = Popen(["vagrant", "ssh", "vmname", "-c", "pvcreate", "-ff", "/dev/sdb"])

And have resorted to using this:

os.system("vagrant ssh vmname -c 'pvcreate -ff /dev/sdb'")

However I need to say yes when it prompts me. I've already tried these options as well:

os.system("yes | vagrant ssh vmname -c 'pvcreate -ff /dev/sdb'")
os.system("echo y | vagrant ssh vmname -c 'pvcreate -ff /dev/sdb'")

Is it possible to respond to a prompt using os.system?