Stop using ROM Manager, it currently doesn't work well with the Optimus V.
Download xionia recovery and place it on your sdcard, just put it on there, don't put it in a folder, it'll be easier.
If you haven't already, install SuperUser by ChainsDD from the market, run it once.
If you haven't already, install Terminal Emulator from the market, open it and type:
flash_image
If you get
flash_image: not found, you need to
download flash_image, unzip it and put it on your sdcard with the xionia recovery file you downloaded earlier.
(If you get
flash_image usage: flash_image partition file.img , skip the next step, it is already installed)
Open Terminal Emulator and type:
su
mount -o remount,rw -t yaffs2 /system /system
cat /sdcard/flash_image > /system/bin/flash_image
chmod 755 /system/bin/flash_image
If you get an error, you will have to use Root Explorer or adb to put flash_image in system/bin, and then just type in terminal emulator:
su
chmod 755 /system/bin/flash_image
If you get through that without any errors, you can then use flash_image in terminal emulator to install the xionia recovery, type:
su
flash_image recovery /sdcard/xionia_cwma_12518.4_virgin.img
Now you can boot into recovery and install any custom ROM.
Be sure to backup first in recovery before you wipe or flash or format anything!