ESXi 5.X doesn't play as nicely with ovf 2.0 elements.
There are two ways to get around this:
1) Import into VMWare Workstation and then Export it to a new OVA.
2) Extract the ova with winzip, or whatever zip program you have. Edit the ovf file to remove the vbox element, then zip it back up (be sure to rename it to .ova)