In principle they could work great together, which demonstrates the almost total lack of overlap.
Ventoy is for making multiboot USB keys.
Plop is for making PCs boot from USB even if their firmware can't.
So, you could use Plop on a floppy or CDR to make a PC start Ventoy, and then use Ventoy to choose which old-style x86/32-BIOS distro to install.
Plop is itself an actual boot manager.
Until you need to boot an old x86 system for some reason