I did a Vanilladpup64 9.2.9 install to a 32GB USB flash drive.
I got it fresh from the Vanilladpup download location.
Used Rufus 3.19 to do install.
When completed used it to boot my computer.
The first boot menu I go was a vanilladpup menu with a lot of selections you could make.
they all looked like different options for how it would boot.
Ram only, no save, etc.....
I selected the first one which was: Vanilladpup64 9.2.9
It started booting and I got that same image as the first one you posted.
The one with some errors about looking for partition iso9660, and some info statements, with waiting 60 seconds for last statement.
After waiting 60 seconds it went to do a normal boot and showing the info statements you normally see for a normal boot.
Info about loading different Vanilladpup SFS files.
Booted to a working desktop.
Typing this from it running my computer.
So something is wrong with your install if you are not seeing the same.
I did get one warning when using Rufus about syslinux and it wanting to use a newer version.
I just did nothing and let it continue.
Made no selection about this warning.
Note:
That 60 seconds delay and error messages is about pmedia=cd in the boot loader menu entry.
There is no iso9660 partition to find because it is not on a CD/DVD.
So it waits 60 seconds and then starts a drive search for the files.
Finds them on the USB and starts a normal boot.
Edited the menu.lst entries and changed all the pmedia= to pmedia=usbflash
Boot menu selecting first option.
No longer boots to the error messages and waits 60 seconds.
Goes directly into the normal boot process info about loading the different SFS files.
Goes start to