15 seconds is for EMMC erasing
And 3 seconds is enough for VIM to enter into upgrade mode.
Do you know how to use USB-to-TTL serial tools, if yes, things would be more easy, as you can also try Serial Mode of following approach:
How to boot into upgrade mode
I guess you didn't got EMMC erased yet, as everything on EMMC will be wiped out after erasing, so your VIM can't display anything on monitor then.
And you need make sure that you have crated the bootable SD card with the ROM image.