Connect a USB pen drive to a Linux OS, in my case a Ubuntu.
    Download the vSphere 5 ESXi ISO installation file.
    On the Linux machine install syslinux and mboot.
    Execute the following commands:

# sudo apt-get install mbr syslinux
# sudo /sbin/fdisk /dev/sdb
# sudo umount /dev/sdb1
# sudo /sbin/mkfs.vfat -F 32 -n USB /dev/sdb1
# cd /tmp/
# sudo syslinux /dev/sdb1
# sudo install-mbr /dev/sdb
# sudo mkdir /mnt/cdrom
# sudo mkdir /mnt/usb
# sudo mount -o loop /tmp/VMware-VMvisor-Installer-5.0.0-469512.x86_64.iso /mnt/cdrom/
# sudo mount /dev/sdb1 /mnt/usb/
# sudo cp -r /mnt/cdrom/* /mnt/usb/
# sudo mv /mnt/usb/isolinux.cfg /mnt/usb/syslinux.cfg
# sudo vi /mnt/usb/syslinux.cfg

In the file /usbdisk/syslinux.cfg, change the line APPEND -c boot.cfg to APPEND -c boot.cfg -p 1.

# sudo umount /mnt/usb
# sudo umount /mnt/cdrom

Ready, says VMware.

But... When I put the USB drive in the server I get the BOOT prompt. The menu is not loaded correctly.

It has got something to do with that this syslinux kernel version needs another layout of the syslinux.cfg file. Untill I solve this issue I just type on the BOOT command prompt:

mboot.c32 -c boot.cfg -p 1

This works.