I have the LG Rumor2 also..But I don't have that problem! Maybe try putting it on ringer because maybe it can't vibrate but it can use ringers while it's charging ! HOPE THIS HELPS
i dont have the rumor2 but i dont think thats supposed to happen with any phone. Like i have the env3 and if i leave my phone on vibrate it stays on vibrate unless i turn it on while its charging and change it. but for the most part mine stays on the mode/volume i left it on when it died.