Sorry, we do not have any experience with VxWorks.
Based on the information I was able to find in internet, to "take the kernel from the EMMC" you have to use fatload command. Depending on image type, if additional files are required (for example device tree file), you have to perform fatload twice. After that to boot the image from memory please use bootm command.
This is based on the information I was able to find by seaching "U-Boot vxworks MMC" keywords, for more precise suggestions please refer to VxWorks documentation and support.
Have a great day,
Alexander
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