There is another way to boot off SD card without having uboot support for it. It involves pin shorting the data pins on the emmc though so you have to be careful. This causes the ATF BL1, which I think is located somewhere else (?), to use SD instead for booting. I have done this on a S912 before and its been a known way to help recover older boxes as well.
I don’t know if there is a safer way to effectively do this (make the emmc temporarily disappear) on a VIM though.
Note: Gouwa’s original comment was about a modified version of uboot that can check the status of the various other states to decide what to boot afterwards. To boot directly off SD without any other support I think you need to have a modified ATF BL1 as noted above, or pin short.
I wrote the following document “[HOW TO RECOVER] So you bricked your box” on freaktab a few months ago, which was covered by a cnx-software article. It applies to the standard amlogic images, so this will probably work on VIM as well. To force the first state in the document you can pin short if its trying to boot off emmc still.