Does the current volume for one's iPhone have any implication on how "loud" it vibrates? Or is vibrate "loudness" static, i.e. the vibrate level remains the same no matter what the phone's volume level is.
Vibrate is the same no matter what the volume level is. You can turn vibrate off for certain contacts if you want which is helpful for "noisy" contacts. Notifications will still show up on the locked screen.