Vim is not handling terminal resizes correctly. You can fix the probelm by starting your xterm with the "-wf" option, like so:
xterm -wf -e vim
Alternatively, you can just cycle through your layouts a few times, which usually seems to fix it.
To see a list of modifier names and their matching keys, use the xmodmap command. On my system, the output looks like this:
> xmodmap xmodmap: up to 3 keys per modifier, (keycodes in parentheses): shift Shift_L (0x32), Shift_R (0x3e) lock Caps_Lock (0x9) control Control_L (0x25), Control_R (0x69) mod1 Alt_L (0x40), Alt_R (0x6c), Meta_L (0xcd) mod2 Num_Lock (0x4d) mod3 mod4 Super_L (0xce), Hyper_L (0xcf) mod5 ISO_Level3_Shift (0x5c), Mode_switch (0xcb)
Copyright (c) 2010 Aldo Cortesi